diff --git a/src/aarecommon/math/diffraction_geometry.py b/src/aarecommon/math/diffraction_geometry.py
index 3bd8e42..2c86afd 100644
--- a/src/aarecommon/math/diffraction_geometry.py
+++ b/src/aarecommon/math/diffraction_geometry.py
@@ -1,11 +1,15 @@
 import math
 from typing import Annotated
 
-from pydantic import BaseModel, Field
+from pydantic import AfterValidator, BaseModel, Field
+
+
+def round_dp(ndigits: int, /) -> AfterValidator:
+    return AfterValidator(lambda v: round(v, ndigits))
 
 
 class DiffractionGeometry(BaseModel):
-    energy_keV: Annotated[float, Field(gt=1.0, lt=100.0)]
+    energy_keV: Annotated[float, Field(gt=1.0, lt=100.0), round_dp(3)]
     dtz_mm: Annotated[float, Field(gt=10.0, lt=5000.0)]
     pixel_size_mm: Annotated[float, Field(ge=0.05, le=0.5)]
     beam_center_pxl: tuple[float, float]