We put our own BMI calculator through a formal accuracy test. We entered 12 real height and weight combinations across US, metric and stone units, worked out the correct answer independently with the exact conversion constants, and compared every number the tool displayed. This page publishes the full results so you can check them, reproduce them, or cite them.
Result: all 12 cases matched on BMI, weight category, BMI Prime and Ponderal Index. Test date: September 26, 2026.
In the interest of transparency: when we first ran this test, 10 of the 12 cases matched. The two that didn’t were rounding-edge cases (a BMI that displays as 25.0 and one that displays as 18.5). We fixed the calculator the same day, re-ran every case, and the numbers below are from that final run. The details are in the boundary section.
How We Tested
- Each input was entered into the live calculator exactly as a visitor would, using the US Units, Metric Units and Other Units tabs.
- Age was set to 30 for every case, so the adult WHO categories apply. (Under 20, the calculator switches to CDC age-and-sex percentiles, which is a different test.)
- The expected answer was calculated separately as weight in kilograms divided by height in meters squared, using the exact constants listed below. No rounding was applied until the final displayed value.
- A case passes only if the displayed BMI, the category, the BMI Prime and the Ponderal Index all match the expected values.
Conversion Constants Used
Most BMI errors come from unit conversion, not from the BMI formula itself. These are the constants the calculator uses. The inch and pound values are exact by international definition.
| Unit | Converted to | Value |
|---|---|---|
| 1 inch | meters | 0.0254 (exact) |
| 1 foot | inches | 12 |
| 1 pound | kilograms | 0.45359237 (exact) |
| 1 stone | kilograms | 6.35029 (14 lb) |
Test Results: 12 Verified Cases
“Exact BMI” is the unrounded value to four decimal places. “Shown” is what the calculator displays. Ponderal Index is weight divided by height cubed, in kg/m³.
| # | Units | Height | Weight | Exact BMI | Shown | Category | BMI Prime | Ponderal Index | Result |
|---|---|---|---|---|---|---|---|---|---|
| 1 | US | 5 ft 9 in | 160 lb | 23.6276 | 23.6 | Normal | 0.95 | 13.5 | Match |
| 2 | US | 5 ft 6 in | 110 lb | 17.7543 | 17.8 | Mild Thinness | 0.71 | 10.6 | Match |
| 3 | US | 5 ft 10 in | 200 lb | 28.6967 | 28.7 | Overweight | 1.15 | 16.1 | Match |
| 4 | US | 6 ft 0 in | 250 lb | 33.9057 | 33.9 | Obese Class I | 1.36 | 18.5 | Match |
| 5 | Metric | 175 cm | 70 kg | 22.8571 | 22.9 | Normal | 0.91 | 13.1 | Match |
| 6 | Metric | 160 cm | 45 kg | 17.5781 | 17.6 | Mild Thinness | 0.70 | 11.0 | Match |
| 7 | Metric | 180 cm | 90 kg | 27.7778 | 27.8 | Overweight | 1.11 | 15.4 | Match |
| 8 | Metric | 165 cm | 95 kg | 34.8944 | 34.9 | Obese Class I | 1.40 | 21.1 | Match |
| 9 | Other (ft/in + stone) | 5 ft 9 in | 11.43 st | 23.6306 | 23.6 | Normal | 0.95 | 13.5 | Match |
| 10 | Other (cm + lb) | 175 cm | 154.3 lb | 22.8536 | 22.9 | Normal | 0.91 | 13.1 | Match |
| 11 | Metric (boundary) | 180 cm | 80.9 kg | 24.9691 | 25.0 | Overweight | 1.00 | 13.9 | Match |
| 12 | Metric (boundary) | 170 cm | 53.4 kg | 18.4775 | 18.5 | Normal | 0.74 | 10.9 | Match |
Cases 9 and 10 are cross-unit checks. Case 9 (11.43 stone) is roughly the same body as case 1 (160 lb), and case 10 (154.3 lb) is roughly the same body as case 5 (70 kg). In both pairs the calculator gives the same displayed BMI regardless of which units you type in.
How Boundary Values Are Handled
BMI categories change at fixed cut-offs: 18.5 (underweight to normal), 25.0 (normal to overweight), 30.0 (overweight to obese). A calculation rarely lands exactly on a cut-off, so the real question is what happens when the unrounded value sits a hair below one.
Case 11 is a good example. The exact BMI is 24.9691, which displays as 25.0. If the category were decided from the unrounded number, the screen would read “25.0 — Normal,” which contradicts the standard WHO table where 25.0 is overweight. Our calculator now assigns the category from the same one-decimal value you see on screen, so 25.0 is always labelled Overweight and 18.5 is always labelled Normal. This was the fix we made during testing.
| Displayed BMI | Category |
|---|---|
| Below 16.0 | Severe Thinness |
| 16.0 – 16.9 | Moderate Thinness |
| 17.0 – 18.4 | Mild Thinness |
| 18.5 – 24.9 | Normal |
| 25.0 – 29.9 | Overweight |
| 30.0 – 34.9 | Obese Class I |
| 35.0 – 39.9 | Obese Class II |
| 40.0 and above | Obese Class III |
Exact Conversion vs. the “× 703” Shortcut
The imperial formula multiplies pounds divided by inches squared by 703. That 703 is the unit-conversion factor rounded to a whole number; carried out in full, the factor is 703.0696. The rounded version reads about 0.01% low.
For 5 ft 9 in and 160 lb, the shortcut gives 23.6253 and the exact conversion gives 23.6276. Both display as 23.6, and both give a BMI Prime of 0.95. In practice the difference only shows up when a result sits within a few thousandths of a rounding edge. Our calculator uses the exact constants, so it never has to lean on the shortcut. For the step-by-step math behind both versions, see BMI Calculation Formula Explained.
A Note on the Healthy Weight Range
Alongside your BMI, the calculator shows a healthy weight range for your height. Its lower end is the weight at BMI 18.5 and its upper end is the weight at BMI 25.0, the exact cut-off where the overweight category begins. For 180 cm that works out to 59.9 kg to 81.0 kg.
Reproduce the Test Yourself
- Open the BMI calculator and enter an age of 30.
- Pick any row above, choose the matching units tab, and enter the height and weight.
- Check the displayed BMI, category, BMI Prime and Ponderal Index against the table.
- To verify the math independently, convert to kilograms and meters with the constants above, then divide weight by height squared.
What This Test Does Not Tell You
This test confirms the calculator does its arithmetic correctly. It says nothing about whether BMI is the right measure for a particular person. BMI can’t tell muscle from fat, so muscular people can read as overweight, and it can understate risk for people who carry fat around the waist. For what each category means in practice, see BMI Categories Explained, and if you want a measure that separates fat from muscle, try our body fat calculator.
How to Cite This Page
You’re welcome to reference these results. A suggested citation:
CalculatorPro. “BMI Calculator Accuracy Test: 12 Verified Examples.” Tested September 26, 2026. https://calculaterpro.com/bmi/bmi-calculator-accuracy-test/
Test Log
- September 26, 2026: Initial run, 10 of 12 cases matched. Fixed category assignment at the 18.5 and 25.0 rounding edges. Re-run: 12 of 12 matched. Also corrected the results gauge so the pointer and colored bands use the same 16 to 40 scale.