People type this into search every day: BMI calculator.
BMI is weight divided by height squared. The body mass index calculator uses the adult form published by the CDC: kilograms divided by metres squared. The number is a screening measure for adults 20 and older. It is not a diagnosis, and it is not a plan. The CDC says to read it with other factors, and to talk with a clinician if you have questions about it. This page shows the arithmetic.
Weight, divided by height squared
BMI = kilograms ÷ (centimetres ÷ 100)²
The calculator then rounds the result to one decimal for the primary number, and it assigns a CDC adult category from the unrounded value.
| Unrounded BMI | Category on this calculator |
|---|---|
| under 18.5 | Underweight |
| 18.5 up to, but not including, 25 | Healthy weight |
| 25 up to, but not including, 30 | Overweight |
| 30 up to, but not including, 35 | Obesity, Class 1 |
| 35 up to, but not including, 40 | Obesity, Class 2 |
| 40 and over | Obesity, Class 3 |
Those cutoffs match the CDC adult BMI categories: under 18.5, 18.5 to less than 25, 25 to less than 30, and 30 or greater, with class 1, class 2, and class 3 inside the obesity range. CDC limits that table to adults 20 and older. For ages 2 through 19, CDC points to a child and teen calculator that uses a percentile, not these adult labels. This tool does not compute that percentile.
The same CDC page says BMI is a screening measure and should be read with other factors. Muscle, bone, and where fat sits can change health risk while this ratio barely moves. The calculator’s own note says the same thing: talk with a clinician before treating the number as a health conclusion.
Fig 1
Kilograms over metres squared
The category uses the unrounded ratio
Height in centimetres becomes metres. BMI is weight divided by that height squared. The adult category follows the CDC cutoffs.
A daily energy estimate from the same height and weight is the calorie post. A lifting estimate is the one-rep max post. Neither one changes this ratio.
Example 1: 175 cm and 70 kg
Height: 175 cm, which is 1.75 m.
Weight: 70 kg.
Square: 1.75 × 1.75 = 3.0625
BMI: 70 ÷ 3.0625 = 22.857, which displays as 22.9
Category: Healthy weight. 22.857 is above 18.5 and below 25.
The card shows the height as 68.9 in and the weight as 154.3 lb. The ratio itself was computed in metres and kilograms. The detail “Height used in the formula” reads 1.75 m.
Calculator result
Body mass index
- Height
- 175 cm
- Weight
- 70 kg
22.9
BMI, CDC adult healthy weight range
- CDC adult category
- Healthy weight
- Height
- 68.9 in
- Weight
- 154.3 lb
- Height used in the formula
- 1.75 m
CDC’s categories page also publishes a worked table for an adult who is 5 feet 9 inches, which is 69 inches. On that table, 125 to 168 pounds falls in the healthy-weight band. Our default displays as 68.9 inches and 154.3 pounds. The height is a tenth of an inch under that published example, and the weight sits inside the band CDC printed for 5 feet 9 inches. Example 6 uses the published height exactly, because the tenth of an inch matters at a cutoff.
Example 2: the cutoffs at the same 175 cm
Hold height at 175 cm (68.9 in on the card). The weight that lands on each cutoff is the BMI times 3.0625.
| BMI | Weight you type | Displayed weight | Category |
|---|---|---|---|
| 18.5 | 56.65625 kg | 124.9 lb | Healthy weight |
| 25 | 76.5625 kg | 168.8 lb | Overweight |
| 30 | 91.875 kg | 202.5 lb | Obesity, Class 1 |
| 35 | 107.1875 kg | 236.3 lb | Obesity, Class 2 |
| 40 | 122.5 kg | 270.1 lb | Obesity, Class 3 |
The boundary belongs to the higher band when the ratio is exact. 18.5 is healthy weight, because the underweight test is “under 18.5”. 25 is overweight. 30 is class 1. 35 is class 2. 40 is class 3.
A weight a hair under the line stays in the lower band, even when the rounded primary looks like the cutoff. Example 6 is that case at 25. The bars below are the displayed pounds at the five exact cutoffs, for this one height.
Fig 2 · 1/5
Pounds at the cutoffs, 175 cm
Each bar is the weight that lands on that BMI
Weight at the cutoff
18.5
124.9 lb
25
168.8 lb
30
202.5 lb
35
236.3 lb
40
270.1 lb
18.5 BMI is 124.9 lb. 25 BMI is 168.8 lb.
30 BMI is 202.5 lb. 40 BMI is 270.1 lb.
Example 3: a shorter adult and a taller one, same style of inputs
160 cm and 85 kg.
Metres: 1.60. Square: 2.56.
BMI: 85 ÷ 2.56 = 33.203, displayed as 33.2
Category: Obesity, Class 1. The ratio is at least 30 and under 35.
The card shows 63 in and 187.4 lb.
Calculator result
Body mass index
- Height
- 160 cm
- Weight
- 85 kg
33.2
BMI, CDC adult obesity range
- CDC adult category
- Obesity
- CDC obesity class
- Class 1
- Height
- 63 in
- Weight
- 187.4 lb
190 cm and 70 kg. Same 70 kg as Example 1, 15 cm taller.
Metres: 1.90. Square: 3.61.
BMI: 70 ÷ 3.61 = 19.391, displayed as 19.4
Category: Healthy weight.
The card shows 74.8 in and the same 154.3 lb.
The 15 cm of height moved the ratio from 22.9 to 19.4. Both are in the healthy-weight band. The shorter, heavier pair in this example is in class 1. Same formula, different inputs, different band.
Fig 3 · 1/3
Three adults, one formula
Unrounded values display to one decimal
BMI
- BMI
| Category | BMI |
|---|---|
| 165 cm, 50 kg | 18.4 |
| 175 cm, 70 kg | 22.9 |
| 160 cm, 85 kg | 33.2 |
175 cm and 70 kg is BMI 22.9.
165 cm and 50 kg is 18.4. 160 cm and 85 kg is 33.2.
The 165 cm, 50 kg column is Example 8’s underweight case, parked here so the three bands share one axis. Its working is in Example 8.
Example 4: what if the scale moves by 5 kg
Height stays 175 cm. Each step is 5 kg. The category uses the unrounded ratio.
| Weight | Displayed | BMI shown | Category |
|---|---|---|---|
| 60 kg | 132.3 lb | 19.6 | Healthy weight |
| 65 kg | 143.3 lb | 21.2 | Healthy weight |
| 70 kg | 154.3 lb | 22.9 | Healthy weight |
| 75 kg | 165.3 lb | 24.5 | Healthy weight |
| 80 kg | 176.4 lb | 26.1 | Overweight |
| 85 kg | 187.4 lb | 27.8 | Overweight |
| 90 kg | 198.4 lb | 29.4 | Overweight |
| 95 kg | 209.4 lb | 31 | Obesity, Class 1 |
From 60 kg to 95 kg the ratio climbs by about 1.6 BMI per 5 kg, because 5 ÷ 3.0625 = 1.633. The displayed steps are 1.6, 1.7, 1.6, 1.6, 1.7, 1.6, 1.6. Rounding to one decimal is why they are not identical.
The band change on this list is between 75 kg and 80 kg. At 75 kg the ratio is 24.49, still under 25. At 80 kg it is 26.12. Five kilograms, displayed as 176.4 − 165.3 = 11.1 lb, is the step that leaves the healthy-weight band in this table. The next band change is between 90 kg (29.39, overweight) and 95 kg (31.02, class 1).
Fig 4 · 1/8
BMI as weight rises, at 175 cm
Each point is 5 kg. Height stays 175 cm.
BMI
- BMI
| Point | BMI |
|---|---|
| 60 kg | 19.6 |
| 65 | 21.2 |
| 70 | 22.9 |
| 75 | 24.5 |
| 80 | 26.1 |
| 85 | 27.8 |
| 90 | 29.4 |
| 95 | 31 |
At 70 kg, BMI is 22.9. At 75 kg it is still 24.5.
At 80 kg, BMI is 26.1. At 95 kg it is 31.
Example 5: what if the height is off by 2 cm
Keep 70 kg. Change only the height. Shoes, hair, and a slouch all live in this 2 cm.
| Height | Displayed | Metres squared | BMI shown | Category |
|---|---|---|---|---|
| 173 cm | 68.1 in | 2.9929 | 23.4 | Healthy weight |
| 175 cm | 68.9 in | 3.0625 | 22.9 | Healthy weight |
| 177 cm | 69.7 in | 3.1329 | 22.3 | Healthy weight |
Working the ends:
- 70 ÷ (1.73 × 1.73) = 70 ÷ 2.9929 = 23.389, displayed as 23.4
- 70 ÷ (1.77 × 1.77) = 70 ÷ 3.1329 = 22.343, displayed as 22.3
Two centimetres of extra height, at this weight, lowers the displayed BMI by 22.9 − 22.3 = 0.6. Two centimetres missing raises it by 23.4 − 22.9 = 0.5. The category stays healthy weight across this particular trio. Near a cutoff, the same 2 cm can change the band, because Example 4 needed only part of a BMI point to cross 25.
Measure without shoes if you want the height the CDC method is built on. The calculator does not know whether the centimetres include a heel.
Example 6: the screen says 25, and the band is still healthy weight
Type 76.5 kg at 175 cm.
Ratio: 76.5 ÷ 3.0625 = 24.980, which is under 25.
Displayed BMI: 25
Category: Healthy weight
Displayed weight: 168.7 lb
The primary number rounded up. The category did not, because 24.980 is still under 25.
Now type the exact cutoff, 76.5625 kg, at the same height.
Ratio: 25 exactly
Displayed BMI: 25
Category: Overweight
Displayed weight: 168.8 lb
The two cards can both say 25. The category line is the one that followed the cutoff. The difference in weight on the screen is 0.1 lb.
CDC’s published table for an adult who is 5 feet 9 inches puts 169 pounds in the overweight band (169 to 202 pounds). Type that height and a matching weight into this calculator:
- height: 175.26 cm, which is 69 × 2.54
- weight: 76.66 kg
The card shows 69 in and 169 lb. The primary number is 25. The category is Healthy weight.
The ratio is 76.66 ÷ (1.7526 × 1.7526) = 24.958. That is under 25, so the adult category on this tool stays in the healthy-weight range. CDC’s pound table for that height calls 169 pounds overweight. The two are using the same cutoffs and a different rounding step. This calculator categorizes the full ratio, then rounds the number you read. A table that rounds to one decimal first can place 25.0 in the overweight band while this screen still says healthy weight.
One more trap on the same card. The detail “Height used in the formula” prints 1.75 m, because it rounds 1.7526 to two decimals. The division used 1.7526. If you recompute 76.66 ÷ (1.75 × 1.75) you get 25.028, and that value is overweight. The card is not using the rounded metres. Use the centimetres you typed, divide by 100, and square that full result.
Calculator result
Body mass index
- Height
- 175.26 cm
- Weight
- 76.66 kg
25
BMI, CDC adult healthy weight range
- CDC adult category
- Healthy weight
- Height
- 69 in
- Weight
- 169 lb
- Height used in the formula
- 1.75 m
Example 7: the common mistakes
Start from Example 1, where the honest ratio is 22.9.
Forgetting the square
70 ÷ 1.75 = 40. A BMI of 40 is the class 3 line. The squared height is 3.0625, not 1.75, and 70 ÷ 3.0625 is 22.9. The missing square is the whole error.
Leaving the height in centimetres
70 ÷ (175 × 175) = 70 ÷ 30,625 = 0.0023. The formula divides centimetres by 100 before it squares. Skip that and the result is a thousand times too small. 1.75 squared is 3.0625. 175 squared is 30,625.
Reading the pounds and inches as if they were the inputs
The card shows 154.3 lb and 68.9 in for the default adult. Dividing 154.3 by 68.9 squared is 154.3 ÷ 4,747.21 = 0.033. That number is not this calculator’s BMI. The calculator already did the metric division. The pounds and inches are a display of the same inputs.
Using adult labels for a child
CDC’s categories page limits this table to age 20 and older, and it sends ages 2 through 19 to a percentile calculator. This tool will still divide any kilograms by any metres squared. The category words it attaches are the adult ones. For a child or a teen, stop at the ratio and use the percentile chart CDC points to.
Example 8: under 18.5, class 2, and class 3
165 cm and 50 kg.
Metres: 1.65. Square: 2.7225.
BMI: 50 ÷ 2.7225 = 18.365, displayed as 18.4
Category: Underweight. 18.365 is under 18.5.
Displayed: 65 in, 110.2 lb.
168 cm and 110 kg.
Metres: 1.68. Square: 2.8224.
BMI: 110 ÷ 2.8224 = 38.974, displayed as 39
Category: Obesity, Class 2. The ratio is at least 35 and under 40.
Displayed: 66.1 in, 242.5 lb.
183 cm and 140 kg.
Metres: 1.83. Square: 3.3489.
BMI: 140 ÷ 3.3489 = 41.805, displayed as 41.8
Category: Obesity, Class 3. The ratio is at least 40.
Displayed: 72 in, 308.6 lb.
Same two fields in every row. The band is only the cutoff table from the top of the page. Class 2 and class 3 are still the obesity range on the CDC page. They are a finer split, not a second formula.
The numbers next to each other
| Example | Height | Weight | BMI shown | Category |
|---|---|---|---|---|
| 1. Defaults | 175 cm | 70 kg | 22.9 | Healthy weight |
| 2. At 18.5 | 175 cm | 56.65625 kg | 18.5 | Healthy weight |
| 2. At 25 | 175 cm | 76.5625 kg | 25 | Overweight |
| 2. At 30 | 175 cm | 91.875 kg | 30 | Obesity, Class 1 |
| 2. At 35 | 175 cm | 107.1875 kg | 35 | Obesity, Class 2 |
| 2. At 40 | 175 cm | 122.5 kg | 40 | Obesity, Class 3 |
| 3. Shorter | 160 cm | 85 kg | 33.2 | Obesity, Class 1 |
| 3. Taller | 190 cm | 70 kg | 19.4 | Healthy weight |
| 4. Five kg under | 175 cm | 65 kg | 21.2 | Healthy weight |
| 4. Five kg over the line | 175 cm | 80 kg | 26.1 | Overweight |
| 5. Height minus 2 cm | 173 cm | 70 kg | 23.4 | Healthy weight |
| 5. Height plus 2 cm | 177 cm | 70 kg | 22.3 | Healthy weight |
| 6. Rounds to 25 | 175 cm | 76.5 kg | 25 | Healthy weight |
| 6. CDC 5 ft 9 in, 169 lb | 175.26 cm | 76.66 kg | 25 | Healthy weight |
| 7. Forgot the square | 175 cm | 70 kg | 40 by hand | the tool still says 22.9 |
| 8. Under 18.5 | 165 cm | 50 kg | 18.4 | Underweight |
| 8. Class 2 | 168 cm | 110 kg | 39 | Obesity, Class 2 |
| 8. Class 3 | 183 cm | 140 kg | 41.8 | Obesity, Class 3 |
Open the body mass index calculator with height in centimetres and weight in kilograms, measured without shoes. Read the category from the unrounded cutoff, especially when the big number lands on 18.5, 25, 30, 35, or 40. For a calorie estimate from the same body, the daily calorie calculator is the next page. For a lift, the one-rep max calculator is a different question. If the person is under 20, use a pediatric percentile chart. CDC’s adult labels are for adults 20 and older, and they remain a screen.