Calculated result
15ft
Diagonal
Formula
UTIL-345-001diagonal = √(A² + B²)
12 ft
Side A
9 ft
Side B
1 yes=1
3-4-5 scaled
Utilities calculator
3-4-5 square calculator
sides 12 ft and 9 ft return a 15 ft diagonal, which is a 3-4-5 scaled by 3. The same default inputs in metric give 4.57 m.
Formula. diagonal = √(A² + B²)
Diagonal of a rectangle so a layout can be squared in the field. Planning geometry only.
By Feroz Sheikh · Formula UTIL-345-001 v1.0.0 · Tested formula
Formula
UTIL-345-001v1.0.0Project inputs
Enter your measurements. Switching units converts the same project, including coverage and unit rates. Package yields keep the unit printed beside each field.
Refine measurements and rates at any time. Your current values are included in the material-list PDF.
Planning estimate, not engineering advice. Confirm site measurements and product specifications. Calculation limits.
Diagonal equals √(A² + B²). A 3-4-5 layout is the same triangle scaled. Planning geometry only — not a survey.
- Pull A and B along the two walls or form boards.
- The diagonal must match before you pour or frame.
- Split odd rooms into rectangles and check each one.
Add your project costs
Add rates if you are pricing a related project line. Pick a currency — badges, inputs, and totals follow.
This page is geometry, not a material takeoff. Leave rates at zero unless you are pricing a related line. Construclytic does not fetch supplier prices.
How this calculator works
A transparent 3-4-5 check square takeoff.
Feroz SheikhWritten and reviewed by Feroz Sheikh · founder
Diagonal equals √(A² + B²). A 3-4-5 layout is the same triangle scaled. Planning geometry only — not a survey.
UTIL-345-001 is the diagonal of a rectangle: √(A² + B²). A 12 and 9 layout is a 3-4-5 scaled by 3, so the diagonal is 15. Both diagonals of a rectangle must match before you pour or frame. Planning geometry only — not a survey.
Worked example. Example: sides 12 ft and 9 ft return a 15 ft diagonal, which is a 3-4-5 scaled by 3.
See the example inputs and calculated result
This separate example uses the calculator's starting values in imperial units. It stays unchanged when you edit the tool above. Example rates, yields and dimensions are assumptions, not recommendations for your project.
| Input | Example value |
|---|---|
| Side A | 12 ft |
| Side B | 9 ft |
Example result: Diagonal — 15 ft.
diagonal = √(A² + B²)
Use the labelled units above when reproducing this example. To work in metric, switch the calculator to Metric; applicable measurements are converted, while product-specific units remain labelled.
- 1. Pull A and B along the two walls or form boards.
- 2. The diagonal must match before you pour or frame.
- 3. Split odd rooms into rectangles and check each one.
Field notes that change the number
- Pull A and B along the two walls or form boards.
- The diagonal must match before you pour or frame.
- Split odd rooms into rectangles and check each one.
Common mistakes
- Treating a short diagonal as close enough and pouring anyway.
- Calling this a survey or a property-line check.
- Mixing metres and feet on the two sides.
Need the broader workflow? Read the utilities planning guide.
How-to: How to check a rectangular layout with diagonals and 3-4-5 · Slope percent, ratio and degrees: convert the same rise and run
Questions
Questions about 3-4-5 check square
What if the diagonal is short?
The corner is not square. Kick a wall until both diagonals of a rectangle match.
Is this a survey?
No. Tape and a calculator. Hire a survey when the property line matters.
What is the formula ID?
This 3-4-5 square calculator publishes formula UTIL-345-001, version 1.0.0. The ID stays on the result and on the estimate line.