diff --git a/cad_core/units.py b/cad_core/units.py
new file mode 100644
index 0000000..74a0566
--- /dev/null
+++ b/cad_core/units.py
@@ -0,0 +1,47 @@
+from __future__ import annotations
+
+from decimal import ROUND_HALF_UP, Decimal
+
+MM_PER_INCH = 25.4
+
+# Global default tolerance for geometric comparisons
+EPS: float = 1e-9
+
+
+def mm_to_inch(mm: float) -> float:
+    return mm / MM_PER_INCH
+
+
+def inch_to_mm(inch: float) -> float:
+    return inch * MM_PER_INCH
+
+
+def almost_equal(a: float, b: float, tol: float = EPS) -> bool:
+    return abs(a - b) <= tol
+
+
+def clamp(x: float, lo: float, hi: float) -> float:
+    if lo > hi:
+        lo, hi = hi, lo
+    return hi if x > hi else lo if x < lo else x
+
+
+def sgn(x: float, tol: float = EPS) -> int:
+    if x > tol:
+        return 1
+    if x < -tol:
+        return -1
+    return 0
+
+
+def round_tol(value: float, tol: float = 1e-6) -> float:
+    if tol <= 0:
+        return value
+    # Try decimal quantization for decimal-friendly tolerances to avoid FP drift
+    try:
+        dval = Decimal(str(value))
+        dtol = Decimal(str(tol))
+        q = (dval / dtol).to_integral_value(rounding=ROUND_HALF_UP)
+        return float(q * dtol)
+    except Exception:
+        return round(value / tol) * tol
diff --git a/tasks/chore-cad-core-units-tolerances-tests.md b/tasks/chore-cad-core-units-tolerances-tests.md
new file mode 100644
index 0000000..bab04f2
--- /dev/null
+++ b/tasks/chore-cad-core-units-tolerances-tests.md
@@ -0,0 +1,17 @@
+Task: cad_core – Units tolerances tests
+
+Scope
+- Add focused tests for `cad_core/units.py`: `EPS`, `almost_equal`, `clamp`, `sgn`, `round_tol`.
+
+Details
+- New file: `tests/cad_core/test_units_tolerances.py` with small, isolated cases.
+- Ensure no dependency on UI or other modules; pure functions only.
+- Keep tolerance behavior consistent across core geometry.
+
+Acceptance
+- `pytest -q` passes; new tests cover edge cases (negative, swapped bounds, zero tol).
+- `ruff` and `black --check` pass.
+
+Branch
+- `chore/cad-core-units-tolerances-tests`
+
diff --git a/tasks/pr/chore-cad-core-units-tolerances-tests.md b/tasks/pr/chore-cad-core-units-tolerances-tests.md
new file mode 100644
index 0000000..dc2ff3d
--- /dev/null
+++ b/tasks/pr/chore-cad-core-units-tolerances-tests.md
@@ -0,0 +1,29 @@
+Title: chore(cad_core): add units tolerances tests
+
+Summary
+- Adds focused tests for `cad_core/units.py` covering `EPS`, `almost_equal`, `clamp`, `sgn`, and `round_tol`.
+- Introduces `cad_core/units.py` helpers with improved `round_tol` using Decimal to avoid FP drift.
+
+Changes
+- New: `tests/cad_core/test_units_tolerances.py`
+- New: `cad_core/units.py`
+- Task: `tasks/chore-cad-core-units-tolerances-tests.md`
+
+Rationale
+- Establish consistent tolerance behavior used across core geometry.
+- Provide small, pure tests to keep regressions visible and fast.
+
+Test Plan (agents pulling this)
+- From repo root:
+  - `python -m pip install -e .` (or set `PYTHONPATH` to repo root)
+  - `ruff check cad_core tests/cad_core/test_units_tolerances.py`
+  - `black --check cad_core tests/cad_core/test_units_tolerances.py`
+  - `pytest -q tests/cad_core/test_units_tolerances.py`
+
+Notes
+- No UI impact; pure functions only.
+- Keep `EPS` and helpers as the single source of tolerance logic across CAD core.
+
+Refs
+- Issue: N/A (please update if applicable)
+
diff --git a/tests/cad_core/test_units_tolerances.py b/tests/cad_core/test_units_tolerances.py
new file mode 100644
index 0000000..22cdfe3
--- /dev/null
+++ b/tests/cad_core/test_units_tolerances.py
@@ -0,0 +1,37 @@
+from cad_core.units import EPS, almost_equal, clamp, round_tol, sgn
+
+
+def test_eps_value_and_usage():
+    assert isinstance(EPS, float)
+    assert EPS == 1e-9
+    # Values within EPS are considered equal
+    assert almost_equal(1.0, 1.0 + EPS * 0.5)
+
+
+def test_almost_equal_with_custom_tol():
+    assert almost_equal(0.0, 1e-6, tol=1e-5)
+    assert not almost_equal(0.0, 2e-5, tol=1e-5)
+
+
+def test_clamp_basic_and_swapped_bounds():
+    assert clamp(5.0, 0.0, 10.0) == 5.0
+    assert clamp(-1.0, 0.0, 10.0) == 0.0
+    assert clamp(11.0, 0.0, 10.0) == 10.0
+    # Swapped bounds should be handled
+    assert clamp(5.0, 10.0, 0.0) == 5.0
+
+
+def test_sgn_with_tolerance():
+    assert sgn(1.0) == 1
+    assert sgn(-1.0) == -1
+    assert sgn(EPS * 0.5) == 0
+    assert sgn(-EPS * 0.5) == 0
+
+
+def test_round_tol_behaviour():
+    assert round_tol(1.2345, 0.01) == 1.23
+    assert round_tol(-1.234, 0.1) == -1.2
+    # Non-positive tolerance returns the input as-is
+    x = 3.14159
+    assert round_tol(x, 0.0) == x
+    assert round_tol(x, -0.1) == x