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Metric + US customary

Square-Up and Diagonal Checker

Calculate a rectangle’s expected diagonal, compare site measurements with a stated tolerance, scale a 3-4-5 triangle or solve one missing right-triangle side.

Square-Up and Diagonal Checker

SiteCalculators · independent field check

Choose a check

Use one measurement system and consistent measuring points throughout.

No sign-in
Measurement system
Metric: dimensions in m; diagonal tolerance in mm.
Rectangle diagonal check
m
m
m
m
mm
Scalable 3-4-5 triangle

Enter a multiplier. The output gives the 3, 4 and 5 side dimensions in the selected unit.

m
Missing right-triangle side

Enter exactly two sides and leave the required side blank. The hypotenuse is opposite the right angle and must be the longest side.

m
m
m
Dimensions are calculated in this browser and are not uploaded. Your unit preference can be remembered locally on this device.
Field-check limitation: Matching diagonals do not prove that every corner is correct, the sides are straight, or the set-out has the right position and orientation. Check approved dimensions, control, offsets and tolerances independently.

How the rectangle check works

For a true rectangle, the corner-to-corner diagonal follows the Pythagorean theorem. Enter the intended length and width, then compare one or both measured diagonals with the expected value. The tool tests each measured diagonal against the tolerance you enter; when both are provided it also reports their difference.

expected diagonal = √(length² + width²)
diagonal error = measured diagonal − expected diagonal

A pass only means the entered diagonal error is inside the entered tolerance. Select the tolerance from the approved design, project specification and suitable survey or trade procedure. Tape sag, slope, temperature, obstructions and inconsistent hook or pin positions can all affect a field measurement.

Worked 10 m × 5 m example

A 10 m by 5 m rectangle has an expected diagonal of √125 = 11.1803399 m. If a diagonal measures 11.182 m, its signed error is +1.66 mm after rounding. With a ±5 mm tolerance that individual check passes. A second diagonal still adds useful evidence and lets the difference between the two site measurements be reviewed.

Using a 3-4-5 triangle

The 3-4-5 relationship is an exact right-triangle ratio. A multiplier of 2 produces 6-8-10; a multiplier of 1.5 produces 4.5-6-7.5. Mark the two legs from one corner, then adjust until the distance between those marks equals the calculated diagonal. Larger multiples normally reduce the influence of small marking errors, provided the available space and measuring method are suitable.

Limitations and assumptions

The formulas assume two-dimensional straight lengths and a right angle between the legs. They do not correct slope distances, establish design control or verify line, level, plumb, coordinate position, corner movement or dimensional compliance. An equal pair of diagonals can occur in geometry that is not the intended rectangle, so retain independent side, angle and control checks.

Frequently asked questions

Do equal diagonals prove a rectangle is square?

No. Equal diagonals are a useful rectangle check, but they do not by themselves prove that every corner is 90 degrees, every line is straight or the set-out is in the correct position.

How does a 3-4-5 triangle set out a right angle?

Measure three equal units along one leg, four along the other and adjust until the diagonal between those marks is five units. Any consistent multiple such as 6-8-10 works the same way.

Should diagonal tolerance be in millimetres or inches?

Use millimetres with Metric or inches with US customary. The suitable tolerance depends on the approved drawing, work type, dimensions, measuring method and project specification.