Diagonal Tolerance and Measurement Repeatability

STEP 01

Define a project limit before measuring

The tolerance field is your limit for the difference between two diagonals. It is not a tolerance on each diagonal, a door reveal allowance or a universal cabinet specification. Choose it from the project drawing, the downstream fit requirement and a measuring setup capable of resolving that limit. The editable 2 mm sample is an illustration, not a recommended shop standard. A small drawer, large cabinet and decorative frame may have different acceptance requirements, and matching diagonals cannot replace size or hardware checks. Record the chosen limit before adjusting the assembly. Changing a tolerance because a reading failed changes the acceptance decision; it does not change the physical part.

STEP 02

Use uncertainty for each reading

Enter a non-negative ± uncertainty for one diagonal reading. The tool uses the same bound for A and B. Consider repeatability, contact-point ambiguity, scale reading and tool setup when selecting that bound. If the two readings have different known bounds, using the larger bound for both is conservative, though less precise. This is a user-entered worst-case bound, not a statistical confidence interval or a calibration certificate. Repeating a reading helps expose variation but cannot prove that a systematic reference error is absent. If you cannot justify a smaller bound than the result needs, improve the measuring method or leave the outcome borderline. Setting uncertainty to zero asserts ideal readings and deserves a deliberate choice.

STEP 03

Read the possible difference range

Let g = |A − B| and let u be the individual reading uncertainty. The possible absolute difference lies between max(0, g − 2u) and g + 2u. The factor of two allows one diagonal to be over-read while the other is under-read. For A = 1,000 mm and B = 1,002 mm with u = 0.5 mm, the observed difference is 2 mm and the possible range is 1–3 mm. With a 2 mm limit, that range crosses the acceptance boundary, so the result is borderline. This range follows directly from the supplied bounded readings. It makes no assumption about the probability of the endpoints and does not predict the exact physical difference.

STEP 04

Understand the three reading states

“Within” means the complete difference range, including its upper endpoint, is at or below your limit. “Outside” means even the lower endpoint is above your limit. “Borderline” means the range includes acceptable and unacceptable values. With a 2 mm limit and ±0.5 mm readings, an observed 0.5 mm difference gives a 0–1.5 mm range and is within. A 4 mm difference gives a 3–5 mm range and is outside. The 2 mm example gives a 1–3 mm range and remains borderline. Do not turn a borderline result into a pass by rounding the displayed value, ignoring the uncertainty or repeatedly measuring until one favorable pair appears.

STEP 05

Keep units and geometry separate

All saved lengths use millimeters. Switching the display to inches converts the numeric inputs and results while retaining the same physical values. For example, 25.4 mm equals 1 inch, and a 600 × 800 mm intended rectangle has a 1,000 mm reference diagonal. The reference comes from the Pythagorean relation, √(width² + height²). It provides a useful independent comparison with the drawing, but matching that number does not prove the assembly is rectangular. A consistent average offset can signal wrong intended dimensions or mismatched endpoints, and a geometry problem can exist even when the average seems right. Inspect that discrepancy instead of using it as a substitute acceptance test.

STEP 06

Resolve a borderline case with better evidence

Recheck the labeled endpoints, tool support, scale and viewing angle. Repeat both readings and look for variation before moving the assembly. If the setup improves, record the new pair with a justified uncertainty and explain the reference in the record name. If the difference remains outside the project limit, inspect equal sides, joint seating, support and the physical fit requirement. The calculator deliberately provides no universal clamp displacement or force because those depend on construction and material behavior. Preserve earlier snapshots so the decision trail remains visible. A within reading state completes only the measurement comparison; the four separate physical checks must also be confirmed before the record shows a completed review.

Method references

WOOD Magazine: How to square your assembly demonstrates dry clamping and diagonal checks. Kreg: Measuring and layout covers reference tools and assembly checks. The bounded uncertainty formula and three reading states are the model used by this tool; they are not a manufacturer tolerance specification.

Record your next reading

Enter the two diagonals with the project-specific limit, your reading uncertainty and the physical check state. Save separate snapshots for each stage so the change remains visible.