diff --git a/OpenNest.Data/NestDefaults.cs b/OpenNest.Data/NestDefaults.cs
new file mode 100644
index 0000000..3424b05
--- /dev/null
+++ b/OpenNest.Data/NestDefaults.cs
@@ -0,0 +1,233 @@
+using System.Text.Json;
+using System.Text.Json.Serialization;
+using OpenNest.Geometry;
+
+namespace OpenNest.Data;
+
+/// <summary>
+/// Plate/nest defaults persisted to a single JSON file
+/// (by default %APPDATA%\OpenNest\defaults.json), replacing the
+/// .nstdot nest-template mechanism. Loading never throws: a missing,
+/// corrupt, or partially valid file degrades field-by-field to
+/// <see cref="Fallback"/> values so creating a new nest is never blocked.
+/// </summary>
+public sealed class NestDefaults
+{
+    public const int CurrentVersion = 1;
+
+    private static readonly JsonSerializerOptions JsonOptions = new()
+    {
+        WriteIndented = true,
+        PropertyNamingPolicy = JsonNamingPolicy.CamelCase,
+        PropertyNameCaseInsensitive = true,
+        Converters = { new JsonStringEnumConverter(JsonNamingPolicy.CamelCase) },
+    };
+
+    public Units Units { get; set; } = Units.Inches;
+
+    public Size Size { get; set; } = new(100, 100);
+
+    public int Quadrant { get; set; } = 1;
+
+    public double PartSpacing { get; set; } = 1;
+
+    public Spacing EdgeSpacing { get; set; } = new(1, 1, 1, 1);
+
+    /// <summary>
+    /// The built-in defaults used when no file exists and for every field
+    /// that is missing or invalid. Matches the historical
+    /// MainForm.CreateDefaultNest values (units default to Inches; callers
+    /// may override from their own settings).
+    /// </summary>
+    public static NestDefaults Fallback => new();
+
+    /// <summary>%APPDATA%\OpenNest\defaults.json.</summary>
+    public static string DefaultPath =>
+        Path.Combine(
+            Environment.GetFolderPath(Environment.SpecialFolder.ApplicationData),
+            "OpenNest",
+            "defaults.json"
+        );
+
+    /// <summary>
+    /// Loads defaults from <paramref name="path"/>, falling back field by
+    /// field for a missing file, invalid JSON, or invalid values.
+    /// </summary>
+    public static NestDefaults Load(string path) => Load(path, out _);
+
+    /// <summary>
+    /// Loads defaults; <paramref name="fileExisted"/> is true when the file
+    /// was present (even if it failed to parse), so callers can surface a
+    /// warning when a present file did not load cleanly.
+    /// </summary>
+    public static NestDefaults Load(string path, out bool fileExisted)
+    {
+        fileExisted = false;
+        var defaults = Fallback;
+        if (string.IsNullOrWhiteSpace(path) || !File.Exists(path))
+            return defaults;
+
+        fileExisted = true;
+
+        NestDefaultsDto? dto;
+        try
+        {
+            var json = File.ReadAllText(path);
+            dto = JsonSerializer.Deserialize<NestDefaultsDto>(json, JsonOptions);
+        }
+        catch (JsonException)
+        {
+            return defaults;
+        }
+        catch (IOException)
+        {
+            return defaults;
+        }
+
+        if (dto is null)
+            return defaults;
+
+        if (
+            dto.Units is not null
+            && Enum.TryParse<Units>(dto.Units, ignoreCase: true, out var units)
+        )
+            defaults.Units = units;
+
+        if (
+            dto.Size?.Width is { } width
+            && dto.Size.Length is { } length
+            && IsValidSize(width, length)
+        )
+            defaults.Size = new Size(width, length);
+
+        if (dto.Quadrant is { } quadrant && quadrant is >= 1 and <= 4)
+            defaults.Quadrant = quadrant;
+
+        if (dto.PartSpacing is { } partSpacing && IsValidSpacing(partSpacing))
+            defaults.PartSpacing = partSpacing;
+
+        if (
+            dto.EdgeSpacing?.Left is { } left
+            && dto.EdgeSpacing.Bottom is { } bottom
+            && dto.EdgeSpacing.Right is { } right
+            && dto.EdgeSpacing.Top is { } top
+            && IsValidSpacing(left)
+            && IsValidSpacing(bottom)
+            && IsValidSpacing(right)
+            && IsValidSpacing(top)
+        )
+            defaults.EdgeSpacing = new Spacing(left, bottom, right, top);
+
+        return defaults;
+    }
+
+    /// <summary>
+    /// Captures the current units and plate defaults from a nest.
+    /// </summary>
+    public static NestDefaults FromNest(Nest nest)
+    {
+        ArgumentNullException.ThrowIfNull(nest);
+        var plate = nest.PlateDefaults;
+        return new NestDefaults
+        {
+            Units = nest.Units,
+            Size = plate.Size,
+            Quadrant = plate.Quadrant,
+            PartSpacing = plate.PartSpacing,
+            EdgeSpacing = plate.EdgeSpacing,
+        };
+    }
+
+    public void ApplyTo(Nest nest)
+    {
+        ArgumentNullException.ThrowIfNull(nest);
+        nest.Units = Units;
+        var plate = nest.PlateDefaults;
+        plate.Size = Size;
+        plate.Quadrant = Quadrant;
+        plate.PartSpacing = PartSpacing;
+        plate.EdgeSpacing = EdgeSpacing;
+    }
+
+    /// <summary>
+    /// Writes the file (creating the parent directory), retrying briefly on
+    /// IO collisions the same way <see cref="LocalJsonProvider"/> does.
+    /// </summary>
+    public void Save(string path, int maxRetries = 3)
+    {
+        var dto = new NestDefaultsDto
+        {
+            Version = CurrentVersion,
+            Units = Units.ToString().ToLowerInvariant(),
+            Size = new SizeDto { Width = Size.Width, Length = Size.Length },
+            Quadrant = Quadrant,
+            PartSpacing = PartSpacing,
+            EdgeSpacing = new SpacingDto
+            {
+                Left = EdgeSpacing.Left,
+                Bottom = EdgeSpacing.Bottom,
+                Right = EdgeSpacing.Right,
+                Top = EdgeSpacing.Top,
+            },
+        };
+
+        var json = JsonSerializer.Serialize(dto, JsonOptions);
+
+        var directory = Path.GetDirectoryName(Path.GetFullPath(path));
+        if (!string.IsNullOrEmpty(directory))
+            Directory.CreateDirectory(directory);
+
+        for (var attempt = 0; attempt < maxRetries; attempt++)
+        {
+            try
+            {
+                File.WriteAllText(path, json);
+                return;
+            }
+            catch (IOException) when (attempt < maxRetries - 1)
+            {
+                Thread.Sleep(100);
+            }
+        }
+    }
+
+    private static bool IsValidSize(double width, double length) =>
+        !double.IsNaN(width)
+            && !double.IsNaN(length)
+            && !double.IsInfinity(width)
+            && !double.IsInfinity(length)
+            && width > 0
+            && length > 0;
+
+    private static bool IsValidSpacing(double value) =>
+        !double.IsNaN(value) && !double.IsInfinity(value) && value >= 0;
+
+    /// <summary>
+    /// Wire format. Every field is nullable so a partial file merges over
+    /// the fallback field by field; unknown fields (including a future
+    /// higher <c>version</c>) are ignored rather than rejected.
+    /// </summary>
+    private sealed record NestDefaultsDto
+    {
+        public int? Version { get; init; } = CurrentVersion;
+        public string? Units { get; init; }
+        public SizeDto? Size { get; init; }
+        public int? Quadrant { get; init; }
+        public double? PartSpacing { get; init; }
+        public SpacingDto? EdgeSpacing { get; init; }
+    }
+
+    private sealed record SizeDto
+    {
+        public double? Width { get; init; }
+        public double? Length { get; init; }
+    }
+
+    private sealed record SpacingDto
+    {
+        public double? Left { get; init; }
+        public double? Bottom { get; init; }
+        public double? Right { get; init; }
+        public double? Top { get; init; }
+    }
+}
diff --git a/OpenNest.Tests/Data/NestDefaultsTests.cs b/OpenNest.Tests/Data/NestDefaultsTests.cs
new file mode 100644
index 0000000..baec274
--- /dev/null
+++ b/OpenNest.Tests/Data/NestDefaultsTests.cs
@@ -0,0 +1,208 @@
+using OpenNest.Data;
+using OpenNest.Geometry;
+
+namespace OpenNest.Tests.Data;
+
+public class NestDefaultsTests : IDisposable
+{
+    private readonly string _testDir;
+    private readonly string _path;
+
+    public NestDefaultsTests()
+    {
+        _testDir = Path.Combine(Path.GetTempPath(), "OpenNestTests", Guid.NewGuid().ToString());
+        Directory.CreateDirectory(_testDir);
+        _path = Path.Combine(_testDir, "defaults.json");
+    }
+
+    public void Dispose()
+    {
+        if (Directory.Exists(_testDir))
+            Directory.Delete(_testDir, true);
+    }
+
+    [Fact]
+    public void Save_ThenLoad_RoundTrips()
+    {
+        var original = new NestDefaults
+        {
+            Units = Units.Millimeters,
+            Size = new Size(1220, 2440),
+            Quadrant = 3,
+            PartSpacing = 2.5,
+            EdgeSpacing = new Spacing(1.5, 3, 1.5, 3),
+        };
+
+        original.Save(_path);
+        var loaded = NestDefaults.Load(_path, out var loadedFromDisk);
+
+        Assert.True(loadedFromDisk);
+        Assert.Equal(Units.Millimeters, loaded.Units);
+        Assert.Equal(original.Size, loaded.Size);
+        Assert.Equal(3, loaded.Quadrant);
+        Assert.Equal(2.5, loaded.PartSpacing);
+        Assert.Equal(original.EdgeSpacing.Left, loaded.EdgeSpacing.Left);
+        Assert.Equal(original.EdgeSpacing.Bottom, loaded.EdgeSpacing.Bottom);
+        Assert.Equal(original.EdgeSpacing.Right, loaded.EdgeSpacing.Right);
+        Assert.Equal(original.EdgeSpacing.Top, loaded.EdgeSpacing.Top);
+    }
+
+    [Fact]
+    public void Save_CreatesMissingDirectory()
+    {
+        var nested = Path.Combine(_testDir, "nested", "defaults.json");
+        new NestDefaults().Save(nested);
+        Assert.True(File.Exists(nested));
+    }
+
+    [Fact]
+    public void Load_MissingFile_ReturnsFallback()
+    {
+        var loaded = NestDefaults.Load(_path, out var loadedFromDisk);
+
+        Assert.False(loadedFromDisk);
+        AssertFallback(loaded);
+    }
+
+    [Fact]
+    public void Load_CorruptJson_ReturnsFallbackButReportsFile()
+    {
+        File.WriteAllText(_path, "{ this is not json");
+
+        var loaded = NestDefaults.Load(_path, out var loadedFromDisk);
+
+        Assert.True(loadedFromDisk);
+        AssertFallback(loaded);
+    }
+
+    [Fact]
+    public void Load_PartialFile_MergesPerField()
+    {
+        File.WriteAllText(_path, """{ "units": "millimeters", "partSpacing": 4 }""");
+
+        var loaded = NestDefaults.Load(_path, out var loadedFromDisk);
+
+        Assert.True(loadedFromDisk);
+        Assert.Equal(Units.Millimeters, loaded.Units);
+        Assert.Equal(4, loaded.PartSpacing);
+        // Untouched fields keep fallback values.
+        Assert.Equal(new Size(100, 100), loaded.Size);
+        Assert.Equal(1, loaded.Quadrant);
+        Assert.Equal(new Spacing(1, 1, 1, 1), loaded.EdgeSpacing);
+    }
+
+    [Fact]
+    public void Load_OutOfRangeValues_FallBackPerField()
+    {
+        File.WriteAllText(
+            _path,
+            """
+            {
+              "units": "furlongs",
+              "size": { "width": -50, "length": 100 },
+              "quadrant": 9,
+              "partSpacing": -1,
+              "edgeSpacing": { "left": 1, "bottom": -2, "right": 1, "top": 1 }
+            }
+            """
+        );
+
+        var loaded = NestDefaults.Load(_path, out var loadedFromDisk);
+
+        Assert.True(loadedFromDisk);
+        Assert.Equal(Units.Inches, loaded.Units);
+        Assert.Equal(new Size(100, 100), loaded.Size);
+        Assert.Equal(1, loaded.Quadrant);
+        Assert.Equal(1, loaded.PartSpacing);
+        Assert.Equal(new Spacing(1, 1, 1, 1), loaded.EdgeSpacing);
+    }
+
+    [Fact]
+    public void Load_NonFiniteValues_FallBack()
+    {
+        File.WriteAllText(_path, """{ "partSpacing": 1e400 }""");
+
+        var loaded = NestDefaults.Load(_path);
+
+        // 1e400 deserializes to Infinity, which is rejected.
+        Assert.Equal(1, loaded.PartSpacing);
+    }
+
+    [Fact]
+    public void Load_UnknownFieldsAndFutureVersion_Ignored()
+    {
+        File.WriteAllText(
+            _path,
+            """
+            {
+              "version": 2,
+              "units": "inches",
+              "partSpacing": 2,
+              "futureFeature": { "enabled": true },
+              "material": { "name": "steel" }
+            }
+            """
+        );
+
+        var loaded = NestDefaults.Load(_path, out var loadedFromDisk);
+
+        Assert.True(loadedFromDisk);
+        Assert.Equal(2, loaded.PartSpacing);
+    }
+
+    [Fact]
+    public void FromNest_CapturesUnitsAndPlateDefaults()
+    {
+        var nest = new Nest
+        {
+            Units = Units.Millimeters,
+        };
+        nest.PlateDefaults.Size = new Size(60, 120);
+        nest.PlateDefaults.Quadrant = 2;
+        nest.PlateDefaults.PartSpacing = 0.5;
+        nest.PlateDefaults.EdgeSpacing = new Spacing(2, 2, 2, 2);
+
+        var captured = NestDefaults.FromNest(nest);
+
+        Assert.Equal(Units.Millimeters, captured.Units);
+        Assert.Equal(new Size(60, 120), captured.Size);
+        Assert.Equal(2, captured.Quadrant);
+        Assert.Equal(0.5, captured.PartSpacing);
+        Assert.Equal(new Spacing(2, 2, 2, 2), captured.EdgeSpacing);
+    }
+
+    [Fact]
+    public void ApplyTo_SetsUnitsAndPlateDefaults_AndDoesNotAliasSourceNest()
+    {
+        var source = new Nest();
+        source.PlateDefaults.Size = new Size(48, 96);
+        source.PlateDefaults.EdgeSpacing = new Spacing(1.25, 1.25, 1.25, 1.25);
+        var defaults = NestDefaults.FromNest(source);
+
+        // Mutating the source nest afterwards must not change the capture.
+        source.PlateDefaults.Size = new Size(1, 1);
+        source.PlateDefaults.EdgeSpacing = new Spacing(9, 9, 9, 9);
+
+        var target = new Nest();
+        defaults.ApplyTo(target);
+
+        Assert.Equal(new Size(48, 96), target.PlateDefaults.Size);
+        Assert.Equal(
+            new Spacing(1.25, 1.25, 1.25, 1.25),
+            target.PlateDefaults.EdgeSpacing
+        );
+
+        // And the applied target owns its own values too.
+        target.PlateDefaults.Size = new Size(2, 2);
+        Assert.Equal(new Size(48, 96), defaults.Size);
+    }
+
+    private static void AssertFallback(NestDefaults loaded)
+    {
+        Assert.Equal(Units.Inches, loaded.Units);
+        Assert.Equal(new Size(100, 100), loaded.Size);
+        Assert.Equal(1, loaded.Quadrant);
+        Assert.Equal(1, loaded.PartSpacing);
+        Assert.Equal(new Spacing(1, 1, 1, 1), loaded.EdgeSpacing);
+    }
+}