How to use the perimeter calculator
Prepare the values for this calculation
Calculate perimeter for a rectangle, circle, or triangle. Use the formula below to check that it answers your question. Read the input units before transferring figures from a statement, measurement or dataset.
Enter and check each input
- Shape: choose Rectangle, Circle, Triangle. Match this selection to the data you enter.
- Primary dimension / side a: enter a value at least 0. Use the unit named in the label or selected above. Do not leave this required field blank.
- Secondary dimension / side b: enter a value at least 0. Use the unit named in the label or selected above. Do not leave this required field blank.
- Side c (triangle only): enter a value at least 0. Use the unit named in the label or selected above. Do not leave this required field blank.
The input table gives a complete example. Keep units and time periods consistent across related fields.
Calculate and resolve any input error
Choose Calculate after entering the values. Changes also update the answer after a short pause. If an error appears, follow its message, correct the field and calculate again. Reset restores the example values.
Interpret the answer
Check Perimeter first. With the unchanged example inputs, it is 18. Interpret it using the formula and assumptions on this page.
Compare and keep a useful result
For a controlled comparison, keep every other example input unchanged and change Primary dimension / side a from 5 to 5.5. The resulting Perimeter is 19. This illustrates the equation’s response to that input, rather than a forecast of real events.
Read the chart’s axis labels and data table. A scenario curve changes one stated input while holding the others fixed; it does not add external observations. Use Download graph to save an available chart as a PNG.
Use Copy result or Download CSV to keep the answer. Share tool shares the page address without placing your input values in the URL. Recheck inputs and assumptions before using the result for a decision.
Perimeter Calculator formula
Rectangle: 2(L+W); circle circumference: 2πr; triangle: a+b+c.
Inputs and units
| Input | Format / range | Example |
|---|---|---|
| Shape | Select an option | Rectangle |
| Primary dimension / side a | minimum 0 | 5 |
| Secondary dimension / side b | minimum 0 | 4 |
| Side c (triangle only) | minimum 0 | 3 |
Worked example
Using the example inputs above, the perimeter is 18.
- Perimeter
- 18
Results and assumptions
The output uses the perimeter convention and formula shown above. Match the entered units and source assumptions when comparing results.
Fix an input or unexpected result
Primary dimension / side a is required. Enter primary dimension / side a using the format and bounds in the input table. Zero and an empty field are different: use zero only if it is allowed and represents your data.
If your answer differs from the example, choose Reset and compare every field, including selected units and options. Check percentage versus decimal entry and whether values describe the same period. Round only the final displayed answer when reproducing the formula.
If a discrepancy remains, report the calculator issue with the page address, non-sensitive example inputs, actual result, expected result and a reference for your calculation.