diff --git a/backend/Domain/Interfaces/IFilamentUsageService.cs b/backend/Domain/Interfaces/IFilamentUsageService.cs
new file mode 100644
index 0000000..1b3a63a
--- /dev/null
+++ b/backend/Domain/Interfaces/IFilamentUsageService.cs
@@ -0,0 +1,50 @@
+using Extrudex.Domain.Entities;
+
+namespace Extrudex.Domain.Interfaces;
+
+/// <summary>
+/// Service for persisting and querying filament usage records.
+/// Tracks consumption per print job and per spool for COGS and inventory tracking.
+/// </summary>
+public interface IFilamentUsageService
+{
+    /// <summary>
+    /// Records a new filament usage entry for a print job.
+    /// </summary>
+    /// <param name="printJobId">The print job that consumed the filament.</param>
+    /// <param name="spoolId">The spool that provided the filament.</param>
+    /// <param name="printerId">The printer that executed the print.</param>
+    /// <param name="gramsUsed">Grams of filament consumed.</param>
+    /// <param name="mmExtruded">Millimeters of filament extruded.</param>
+    /// <param name="notes">Optional notes about this usage record.</param>
+    /// <param name="cancellationToken">Cancellation token.</param>
+    /// <returns>The created FilamentUsage entity.</returns>
+    Task<FilamentUsage> RecordUsageAsync(
+        Guid printJobId,
+        Guid spoolId,
+        Guid printerId,
+        decimal gramsUsed,
+        decimal mmExtruded,
+        string? notes = null,
+        CancellationToken cancellationToken = default);
+
+    /// <summary>
+    /// Retrieves all filament usage records for a specific print job.
+    /// </summary>
+    /// <param name="printJobId">The print job ID.</param>
+    /// <param name="cancellationToken">Cancellation token.</param>
+    /// <returns>Collection of filament usage records for the print job.</returns>
+    Task<IReadOnlyList<FilamentUsage>> GetByPrintJobAsync(
+        Guid printJobId,
+        CancellationToken cancellationToken = default);
+
+    /// <summary>
+    /// Retrieves all filament usage records for a specific spool.
+    /// </summary>
+    /// <param name="spoolId">The spool ID.</param>
+    /// <param name="cancellationToken">Cancellation token.</param>
+    /// <returns>Collection of filament usage records for the spool.</returns>
+    Task<IReadOnlyList<FilamentUsage>> GetBySpoolAsync(
+        Guid spoolId,
+        CancellationToken cancellationToken = default);
+}
\ No newline at end of file
diff --git a/backend/Infrastructure/Services/FilamentUsageService.cs b/backend/Infrastructure/Services/FilamentUsageService.cs
new file mode 100644
index 0000000..c4b95c8
--- /dev/null
+++ b/backend/Infrastructure/Services/FilamentUsageService.cs
@@ -0,0 +1,79 @@
+using Extrudex.Domain.Entities;
+using Extrudex.Domain.Interfaces;
+using Extrudex.Infrastructure.Data;
+using Microsoft.EntityFrameworkCore;
+using Microsoft.Extensions.Logging;
+
+namespace Extrudex.Infrastructure.Services;
+
+/// <summary>
+/// EF Core–backed implementation of the filament usage service.
+/// Persists usage records to the database and provides query methods
+/// for retrieving usage by print job or spool.
+/// </summary>
+public class FilamentUsageService : IFilamentUsageService
+{
+    private readonly ExtrudexDbContext _dbContext;
+    private readonly ILogger<FilamentUsageService> _logger;
+
+    public FilamentUsageService(
+        ExtrudexDbContext dbContext,
+        ILogger<FilamentUsageService> logger)
+    {
+        _dbContext = dbContext;
+        _logger = logger;
+    }
+
+    /// <inheritdoc />
+    public async Task<FilamentUsage> RecordUsageAsync(
+        Guid printJobId,
+        Guid spoolId,
+        Guid printerId,
+        decimal gramsUsed,
+        decimal mmExtruded,
+        string? notes = null,
+        CancellationToken cancellationToken = default)
+    {
+        var usage = new FilamentUsage
+        {
+            PrintJobId = printJobId,
+            SpoolId = spoolId,
+            PrinterId = printerId,
+            GramsUsed = gramsUsed,
+            MmExtruded = mmExtruded,
+            RecordedAt = DateTime.UtcNow,
+            Notes = notes
+        };
+
+        _dbContext.FilamentUsages.Add(usage);
+        await _dbContext.SaveChangesAsync(cancellationToken);
+
+        _logger.LogInformation(
+            "Recorded filament usage: {Grams}g / {Mm}mm for print job {JobId} on spool {SpoolId}",
+            gramsUsed, mmExtruded, printJobId, spoolId);
+
+        return usage;
+    }
+
+    /// <inheritdoc />
+    public async Task<IReadOnlyList<FilamentUsage>> GetByPrintJobAsync(
+        Guid printJobId,
+        CancellationToken cancellationToken = default)
+    {
+        return await _dbContext.FilamentUsages
+            .Where(u => u.PrintJobId == printJobId)
+            .OrderByDescending(u => u.RecordedAt)
+            .ToListAsync(cancellationToken);
+    }
+
+    /// <inheritdoc />
+    public async Task<IReadOnlyList<FilamentUsage>> GetBySpoolAsync(
+        Guid spoolId,
+        CancellationToken cancellationToken = default)
+    {
+        return await _dbContext.FilamentUsages
+            .Where(u => u.SpoolId == spoolId)
+            .OrderByDescending(u => u.RecordedAt)
+            .ToListAsync(cancellationToken);
+    }
+}
\ No newline at end of file
diff --git a/backend/Infrastructure/Services/MoonrakerUsagePoller.cs b/backend/Infrastructure/Services/MoonrakerUsagePoller.cs
new file mode 100644
index 0000000..739bcbd
--- /dev/null
+++ b/backend/Infrastructure/Services/MoonrakerUsagePoller.cs
@@ -0,0 +1,390 @@
+using Extrudex.Domain.DTOs.Moonraker;
+using Extrudex.Domain.Entities;
+using Extrudex.Domain.Enums;
+using Extrudex.Domain.Interfaces;
+using Extrudex.Infrastructure.Data;
+using Microsoft.EntityFrameworkCore;
+using Microsoft.Extensions.Logging;
+using Microsoft.Extensions.Options;
+
+namespace Extrudex.Infrastructure.Services;
+
+/// <summary>
+/// Configuration options for the Moonraker usage polling service.
+/// </summary>
+public class MoonrakerPollerOptions
+{
+    /// <summary>
+    /// How often to poll each Moonraker printer for filament usage data.
+    /// Default: 30 seconds.
+    /// </summary>
+    public TimeSpan PollInterval { get; set; } = TimeSpan.FromSeconds(30);
+
+    /// <summary>
+    /// Timeout for individual Moonraker HTTP requests.
+    /// Default: 10 seconds.
+    /// </summary>
+    public TimeSpan RequestTimeout { get; set; } = TimeSpan.FromSeconds(10);
+
+    /// <summary>
+    /// Whether the polling service is enabled. Default: true.
+    /// Set to false to disable polling (e.g., in development or testing).
+    /// </summary>
+    public bool Enabled { get; set; } = true;
+}
+
+/// <summary>
+/// Background service that periodically polls Moonraker-connected printers
+/// for filament usage data. When a print job is detected as complete,
+/// the usage data is persisted to the FilamentUsage table via
+/// <see cref="IFilamentUsageService"/>.
+///
+/// <para>Polling logic:</para>
+/// <list type="number">
+///   <item>Query the database for all active printers with ConnectionType == Moonraker.</item>
+///   <item>For each printer, call <see cref="IMoonrakerClient.GetPrintStatsAsync"/> for live data
+///         and <see cref="IMoonrakerClient.GetPrintHistoryAsync"/> for completed job history.</item>
+///   <item>If usage data is available and the print state is "complete",
+///         create or update a FilamentUsage record.</item>
+///   <item>If the printer is unreachable or returns malformed data, log a warning
+///         and continue to the next printer (no crash).</item>
+/// </list>
+///
+/// <para>Error handling:</para>
+/// <list type="bullet">
+///   <item>API unreachable: logged as warning, poller continues for other printers.</item>
+///   <item>Malformed response: logged as warning, poller continues.</item>
+///   <item>Database errors: logged as error, poller continues.</item>
+/// </list>
+/// </summary>
+public class MoonrakerUsagePoller : BackgroundService
+{
+    private readonly IServiceScopeFactory _scopeFactory;
+    private readonly ILogger<MoonrakerUsagePoller> _logger;
+    private readonly MoonrakerPollerOptions _options;
+
+    /// <summary>
+    /// Tracks which Moonraker print jobs have already been recorded,
+    /// keyed by "printerId:gcodeFileName" to avoid duplicate recording.
+    /// </summary>
+    private readonly HashSet<string> _recordedJobs = new();
+
+    public MoonrakerUsagePoller(
+        IServiceScopeFactory scopeFactory,
+        ILogger<MoonrakerUsagePoller> logger,
+        IOptions<MoonrakerPollerOptions> options)
+    {
+        _scopeFactory = scopeFactory;
+        _logger = logger;
+        _options = options.Value;
+    }
+
+    protected override async Task ExecuteAsync(CancellationToken stoppingToken)
+    {
+        if (!_options.Enabled)
+        {
+            _logger.LogInformation("Moonraker usage poller is disabled via configuration.");
+            return;
+        }
+
+        _logger.LogInformation(
+            "Moonraker usage poller starting. Poll interval: {Interval}",
+            _options.PollInterval);
+
+        while (!stoppingToken.IsCancellationRequested)
+        {
+            try
+            {
+                await PollAllPrintersAsync(stoppingToken);
+            }
+            catch (Exception ex)
+            {
+                _logger.LogError(ex,
+                    "Unexpected error in Moonraker usage poller cycle. Continuing.");
+            }
+
+            await Task.Delay(_options.PollInterval, stoppingToken);
+        }
+
+        _logger.LogInformation("Moonraker usage poller stopping.");
+    }
+
+    private async Task PollAllPrintersAsync(CancellationToken cancellationToken)
+    {
+        using var scope = _scopeFactory.CreateScope();
+        var dbContext = scope.ServiceProvider.GetRequiredService<ExtrudexDbContext>();
+        var moonrakerClient = scope.ServiceProvider.GetRequiredService<IMoonrakerClient>();
+        var usageService = scope.ServiceProvider.GetRequiredService<IFilamentUsageService>();
+
+        var printers = await dbContext.Printers
+            .Where(p => p.IsActive && p.ConnectionType == ConnectionType.Moonraker)
+            .ToListAsync(cancellationToken);
+
+        if (printers.Count == 0)
+        {
+            _logger.LogDebug("No active Moonraker printers found.");
+            return;
+        }
+
+        _logger.LogDebug("Polling {Count} Moonraker printer(s).", printers.Count);
+
+        foreach (var printer in printers)
+        {
+            await PollPrinterAsync(
+                printer, moonrakerClient, usageService, dbContext, cancellationToken);
+        }
+    }
+
+    private async Task PollPrinterAsync(
+        Printer printer,
+        IMoonrakerClient moonrakerClient,
+        IFilamentUsageService usageService,
+        ExtrudexDbContext dbContext,
+        CancellationToken cancellationToken)
+    {
+        _logger.LogDebug(
+            "Polling Moonraker printer {PrinterName} ({Host}:{Port})",
+            printer.Name, printer.HostnameOrIp, printer.Port);
+
+        try
+        {
+            var printStats = await moonrakerClient.GetPrintStatsAsync(
+                printer.HostnameOrIp,
+                printer.Port,
+                printer.ApiKey,
+                cancellationToken);
+
+            if (printStats is null)
+            {
+                _logger.LogDebug(
+                    "No print stats available from printer {PrinterName}.", printer.Name);
+                return;
+            }
+
+            printer.LastSeenAt = DateTime.UtcNow;
+            await dbContext.SaveChangesAsync(cancellationToken);
+
+            _logger.LogDebug(
+                "Printer {PrinterName}: state={State}, filament={Mm}mm, file={File}",
+                printer.Name, printStats.State, printStats.FilamentUsedMm, printStats.Filename);
+
+            decimal mmExtruded = printStats.FilamentUsedMm;
+            if (mmExtruded <= 0)
+            {
+                _logger.LogDebug(
+                    "Printer {PrinterName} has no filament usage to record.", printer.Name);
+                return;
+            }
+
+            if (!IsCompleteState(printStats.State))
+            {
+                _logger.LogDebug(
+                    "Printer {PrinterName} print state '{State}' is not complete; skipping.",
+                    printer.Name, printStats.State);
+                return;
+            }
+
+            string gcodeFileName = printStats.Filename ?? $"unknown-{Guid.NewGuid():N}";
+            var deduplicationKey = $"{printer.Id}:{gcodeFileName}";
+            if (_recordedJobs.Contains(deduplicationKey))
+            {
+                _logger.LogDebug(
+                    "Printer {PrinterName} job '{File}' already recorded; skipping.",
+                    printer.Name, gcodeFileName);
+                return;
+            }
+
+            DateTime? startedAt = null;
+            DateTime? completedAt = null;
+            try
+            {
+                var history = await moonrakerClient.GetPrintHistoryAsync(
+                    printer.HostnameOrIp, printer.Port, printer.ApiKey,
+                    limit: 1, cancellationToken);
+
+                if (history.Items.Count > 0)
+                {
+                    var latestJob = history.Items[0];
+                    startedAt = latestJob.StartTime;
+                    completedAt = latestJob.EndTime;
+                }
+            }
+            catch (Exception ex)
+            {
+                _logger.LogDebug(ex,
+                    "Could not fetch history for printer {PrinterName}; proceeding with stats only.",
+                    printer.Name);
+            }
+
+            var printJob = await FindOrCreatePrintJobAsync(
+                dbContext, printer, mmExtruded, gcodeFileName,
+                startedAt, completedAt, cancellationToken);
+
+            if (printJob is null)
+            {
+                _logger.LogWarning(
+                    "Could not find or create print job for printer {PrinterName}. No active spool found.",
+                    printer.Name);
+                return;
+            }
+
+            var spool = await dbContext.Spools.FindAsync(
+                new object[] { printJob.SpoolId }, cancellationToken);
+
+            var gramsUsed = CalculateGramsUsed(mmExtruded, spool);
+
+            await usageService.RecordUsageAsync(
+                printJobId: printJob.Id,
+                spoolId: printJob.SpoolId,
+                printerId: printer.Id,
+                gramsUsed: gramsUsed,
+                mmExtruded: mmExtruded,
+                notes: $"Moonraker auto-recorded: {gcodeFileName}",
+                cancellationToken: cancellationToken);
+
+            _recordedJobs.Add(deduplicationKey);
+
+            _logger.LogInformation(
+                "Recorded Moonraker usage for printer {PrinterName}: {Mm}mm / {Grams}g, job '{File}'",
+                printer.Name, mmExtruded, gramsUsed, gcodeFileName);
+        }
+        catch (HttpRequestException ex)
+        {
+            _logger.LogWarning(ex,
+                "Moonraker API unreachable for printer {PrinterName} ({Host}:{Port}). Will retry next cycle.",
+                printer.Name, printer.HostnameOrIp, printer.Port);
+        }
+        catch (TaskCanceledException) when (cancellationToken.IsCancellationRequested)
+        {
+            throw;
+        }
+        catch (TaskCanceledException ex)
+        {
+            _logger.LogWarning(ex,
+                "Moonraker request timed out for printer {PrinterName} ({Host}:{Port}).",
+                printer.Name, printer.HostnameOrIp, printer.Port);
+        }
+        catch (Exception ex)
+        {
+            _logger.LogError(ex,
+                "Unexpected error polling Moonraker printer {PrinterName}. Continuing to next printer.",
+                printer.Name);
+        }
+    }
+
+    private static bool IsCompleteState(string state) =>
+        state.Equals("complete", StringComparison.OrdinalIgnoreCase) ||
+        state.Equals("completed", StringComparison.OrdinalIgnoreCase);
+
+    private async Task<PrintJob?> FindOrCreatePrintJobAsync(
+        ExtrudexDbContext dbContext,
+        Printer printer,
+        decimal mmExtruded,
+        string gcodeFileName,
+        DateTime? startedAt,
+        DateTime? completedAt,
+        CancellationToken cancellationToken)
+    {
+        if (!string.IsNullOrEmpty(gcodeFileName))
+        {
+            var existingJob = await dbContext.PrintJobs
+                .Where(j => j.PrinterId == printer.Id &&
+                            j.GcodeFilePath == gcodeFileName &&
+                            j.DataSource == DataSource.Moonraker &&
+                            j.Status != JobStatus.Cancelled)
+                .OrderByDescending(j => j.CreatedAt)
+                .FirstOrDefaultAsync(cancellationToken);
+
+            if (existingJob is not null)
+            {
+                existingJob.MmExtruded = mmExtruded;
+                existingJob.GramsDerived = CalculateGramsUsed(
+                    mmExtruded,
+                    await dbContext.Spools.FindAsync(
+                        new object[] { existingJob.SpoolId }, cancellationToken));
+                existingJob.Status = JobStatus.Completed;
+                existingJob.CompletedAt = completedAt ?? DateTime.UtcNow;
+                existingJob.StartedAt ??= startedAt;
+                await dbContext.SaveChangesAsync(cancellationToken);
+                return existingJob;
+            }
+        }
+
+        var spool = await FindActiveSpoolForPrinterAsync(dbContext, printer, cancellationToken);
+        if (spool is null) return null;
+
+        var gramsDerived = CalculateGramsUsed(mmExtruded, spool);
+
+        var newJob = new PrintJob
+        {
+            PrinterId = printer.Id,
+            SpoolId = spool.Id,
+            PrintName = gcodeFileName ?? "Moonraker Print",
+            GcodeFilePath = gcodeFileName,
+            MmExtruded = mmExtruded,
+            GramsDerived = gramsDerived,
+            FilamentDiameterAtPrintMm = spool.FilamentDiameterMm,
+            MaterialDensityAtPrint = GetMaterialDensity(spool),
+            DataSource = DataSource.Moonraker,
+            Status = JobStatus.Completed,
+            StartedAt = startedAt ?? DateTime.UtcNow,
+            CompletedAt = completedAt ?? DateTime.UtcNow,
+            Notes = "Auto-created by Moonraker usage poller"
+        };
+
+        dbContext.PrintJobs.Add(newJob);
+        await dbContext.SaveChangesAsync(cancellationToken);
+
+        return newJob;
+    }
+
+    private static async Task<Spool?> FindActiveSpoolForPrinterAsync(
+        ExtrudexDbContext dbContext,
+        Printer printer,
+        CancellationToken cancellationToken)
+    {
+        var amsSpool = await dbContext.AmsSlots
+            .Include(s => s.Spool)
+            .ThenInclude(s => s!.MaterialBase)
+            .Include(s => s.AmsUnit)
+            .Where(s => s.AmsUnit.PrinterId == printer.Id && s.Spool != null && s.Spool.IsActive)
+            .Select(s => s.Spool)
+            .FirstOrDefaultAsync(cancellationToken);
+
+        if (amsSpool is not null) return amsSpool;
+
+        return await dbContext.Spools
+            .Include(s => s.MaterialBase)
+            .Where(s => s.IsActive)
+            .OrderByDescending(s => s.WeightRemainingGrams)
+            .FirstOrDefaultAsync(cancellationToken);
+    }
+
+    private static decimal CalculateGramsUsed(decimal mmExtruded, Spool? spool)
+    {
+        if (spool is null) return 0m;
+        var diameterMm = spool.FilamentDiameterMm;
+        var densityGcm3 = GetMaterialDensity(spool);
+        var radiusMm = diameterMm / 2m;
+        var crossSectionArea = Math.PI * (double)radiusMm * (double)radiusMm;
+        var volumeMm3 = (double)mmExtruded * crossSectionArea;
+        var volumeCm3 = volumeMm3 / 1000.0;
+        var grams = volumeCm3 * (double)densityGcm3;
+        return Math.Round((decimal)grams, 2);
+    }
+
+    private static decimal GetMaterialDensity(Spool? spool)
+    {
+        return spool?.MaterialBase?.Name?.ToUpperInvariant() switch
+        {
+            "PLA" => 1.24m,
+            "PETG" => 1.27m,
+            "ABS" => 1.04m,
+            "ASA" => 1.07m,
+            "TPU" => 1.21m,
+            "NYLON" or "PA" => 1.13m,
+            "PC" => 1.20m,
+            _ => 1.24m
+        };
+    }
+}
\ No newline at end of file
diff --git a/backend/Program.cs b/backend/Program.cs
index d664d0c..4b01947 100644
--- a/backend/Program.cs
+++ b/backend/Program.cs
@@ -61,6 +61,14 @@ builder.Services.AddSingleton<ILowStockDetector, LowStockDetector>();
 // ── Usage Logging ───────────────────────────────────────────
 builder.Services.AddScoped<IUsageLogService, UsageLogService>();
 
+// ── Filament Usage Service ──────────────────────────────────
+builder.Services.AddScoped<IFilamentUsageService, FilamentUsageService>();
+
+// ── Moonraker Usage Poller (Background Service) ─────────────
+builder.Services.Configure<MoonrakerPollerOptions>(
+    builder.Configuration.GetSection("MoonrakerPoller"));
+builder.Services.AddHostedService<MoonrakerUsagePoller>();
+
 // ── FluentValidation ──────────────────────────────────────
 // Registers all validators from the API assembly into DI.
 builder.Services.AddValidatorsFromAssembly(Assembly.GetExecutingAssembly());
diff --git a/backend/appsettings.json b/backend/appsettings.json
index a7c2fa0..00507a0 100644
--- a/backend/appsettings.json
+++ b/backend/appsettings.json
@@ -23,5 +23,11 @@
   },
   "FilamentAlerts": {
     "LowStockThresholdPercent": 20
+  },
+  "MoonrakerPoller": {
+    "Enabled": true,
+    "PollInterval": "00:00:30",
+    "RequestTimeout": "00:00:10"
   }
+}
 }
\ No newline at end of file