· Built around the BMI calculator
Photo: RUN 4 FFWPU on Pexels (free licence)
Late October is when South Africans start counting down to December - and to the beach, the Cape Town Marathon training block or simply fitting into last summer's clothes. Three numbers come up every time: body mass index, how much energy you use in a day, and running pace. This post works each one through the BMI, calories a day and running pace calculators, with real numbers, and shows what each one can and cannot tell you.
Run your own numbers
The BMI calculator starts on the first example below: 70 kg and 175 cm. Type your own weight and height - it shows your BMI, the healthy weight range for your height and how far you are from it.
Checked against a published example
Every calculator in this post was run on the inputs of a published worked example and now gives the same answer:
- BMI: the CDC's table for an adult 5 ft 9 in (175.26 cm) tall puts 124 lb in underweight, 125 to 168 lb healthy, 169 to 202 lb overweight and 203 lb and up in obesity (classes from 237 and 271 lb). Our BMI calculator now puts all ten boundary weights in the same category.
- Calories: the Inch Calculator Mifflin-St Jeor example - a woman of 54.55 kg, 165.1 cm, with 35 used for age - gives a resting rate of 1 241 kcal. Our calories calculator gives 1 241 kcal for the same inputs (that page's text says 36 years old; its sum uses 35).
- Pace: Runna's 3:30 marathon chart gives 4:59 a km, 24:53 at 5 km, 49:46 at 10 km and 1:45:00 at halfway. Our pace calculator gives the same four times for 42.195 km in 3:30:00.
One fix was needed. Our BMI calculator sorted you into a category on the unrounded number while showing one decimal - so 169 lb at 5 ft 9 in showed "BMI 25.0" but "a healthy weight", and 125 lb showed "BMI 18.5" but "underweight". It now uses the BMI it shows, the way the WHO's 18.5 and 24.9 cut-offs and the CDC table read. Results away from a boundary did not change.
Worked example 1: what your BMI says
- BMI = weight in kg / (height in metres x height in metres) (BMI calculator).
- 70 kg at 1.75 m: 1.75 x 1.75 = 3.0625, and 70 / 3.0625 = 22.9. That is in the healthy band, 18.5 to 24.9.
- The healthy range at 1.75 m is 56.7 kg to 76.3 kg - 18.5 x 3.0625 and 24.9 x 3.0625.
- 95 kg at 1.72 m is a BMI of 32.1, in obesity class 1 (30 to 34.9). Reaching the top of the healthy range, 73.7 kg, means losing 21.3 kg.
- A first step counts: 5% of 95 kg is 4.75 kg. Health services commonly treat a 5% loss as a meaningful first target - check with your doctor or clinic.
- BMI cannot tell muscle from fat, so a front-row forward reads "overweight". It is a screening number for adults 18 and over - children are measured on BMI-for-age charts.
Worked example 2: calories and kilojoules a day
- Resting energy (Mifflin-St Jeor) = 10 x kg + 6.25 x cm - 5 x age, plus 5 for men or minus 161 for women (calories calculator).
- Man, 35, 80 kg, 1.80 m: 800 + 1 125 - 175 + 5 = 1 755 kcal at rest.
- On his feet some of the day (activity level 1.55) he uses about 2 720 kcal a day - 1 755 x 1.55. Mostly sitting (1.40) it is 2 457; a physical job (1.85) about 3 247.
- In kilojoules that is 11 382 kJ (1 kcal = 4.184 kJ) - the unit SA food labels lead with.
- Woman, 32, 65 kg, 1.63 m, a desk job: 650 + 1 018.75 - 160 - 161 = 1 348 kcal at rest, and x 1.40 = about 1 887 kcal (7 895 kJ) a day.
- The same woman training two or three times a week (1.55) uses about 2 089 kcal - roughly 200 kcal more, about a slice of bread with peanut butter.
- These are estimates for healthy adults, not for pregnancy, teenagers or medical diets. A dietitian can tailor them.
Photo: RUN 4 FFWPU on Pexels (free licence)
Worked example 3: what your parkrun pace means
- Pace = time / distance, shown as minutes and seconds a kilometre; speed = distance / hours (pace calculator).
- A 30-minute parkrun is 6:00 a km, or 10.0 km/h - 9:39 a mile.
- Held all the way, 6:00 a km is a 1:00:00 10 km, a 2:06:35 half marathon and a 4:13:10 marathon.
- A 25-minute parkrun is 5:00 a km - a 1:45:29 half and a 3:30:58 marathon at the same pace.
- To break 3:30 in a marathon you need 4:59 a km - one second a km faster than a 25-minute parkrun, for 42.2 km instead of 5.
- Most runners slow over longer distances, and hills and heat cost more - so treat these as target paces, not predictions.
Worked example 4: putting the three together
- The 95 kg, 1.72 m runner above, aged 40 and training three times a week, uses about 2 837 kcal a day: 950 + 1 075 - 200 + 5 = 1 830 at rest, x 1.55 (calories calculator).
- Eating about 500 kcal a day less than that is a common starting deficit - roughly 2 337 kcal, or 9 778 kJ.
- As weight comes off, the number falls: at 85 kg the same man uses about 2 682 kcal, so re-run the calculator every few kilos.
- Track pace, not just the scale: the same parkrun getting faster is progress the BMI cannot show.
Not verified from a primary source: the 500 kcal-a-day deficit and the 5% first target are common clinical rules of thumb, not figures from a South African health department publication. The Inch Calculator and Runna examples are calculators' own published examples, not standards. None of this is medical advice - see a doctor before a big change in diet or training, especially with a heart condition, diabetes or high blood pressure.
What to do this month
- Work out your BMI and healthy range once, then watch the trend rather than the day-to-day number (BMI calculator).
- Read your daily kilojoules against food labels for a week before cutting anything (calories calculator).
- Set a race pace from your parkrun time and check it every month (pace calculator).
Sources
- BMI categories and the 5 ft 9 in example table: CDC, Adult BMI Categories, read by CalcSA 16 October 2026.
- Mifflin-St Jeor worked example: Inch Calculator, Mifflin St. Jeor Calculator, read 16 October 2026. Equation: Mifflin MD, St Jeor ST et al., American Journal of Clinical Nutrition 1990; 51:241-247.
- Activity levels: FAO/WHO/UNU, Human Energy Requirements (2001 expert consultation report).
- 3:30 marathon splits: Runna, 3:30 Marathon Pace Chart, read 16 October 2026. Marathon 42.195 km: World Athletics.
Calculators used in this post
← All posts · Open the BMI calculator