# Body Fat Calculator | Forge > **Quick Answer:** The Forge Body Fat Calculator estimates your body fat percentage at home. Use the US Navy tape method with a measuring tape, or a quick estimate from height, weight, age, and sex. You also get fat mass, lean mass, and your category on the ACE chart. Free and ungated. Example: a 70-inch-tall man with a 38-inch waist and 16-inch neck lands near 23%. **dateModified:** 2026-08-09 --- ## Key Facts - **Two methods:** US Navy tape method (Hodgdon-Beckett) or the Deurenberg height-and-weight estimate - **Tape sites:** Men measure waist at the navel and neck below the larynx. Women add hips at the widest point and measure the waist at the narrowest point. - **Outputs:** Body fat percentage, fat mass, lean mass, and ACE category - **Accuracy:** Within three to four percentage points of a DEXA scan with careful tape work - **Units:** Imperial and metric - **Cost:** Free. No sign-up. --- ## How to Use It 1. Pick the Navy method if you own a tape measure. Use the quick estimate when you have nothing to measure with. 2. Enter your sex, height, and weight. 3. For the Navy method, measure the tape sites level and snug, breathing out normally. Take each measurement twice. 4. Read your percentage, fat mass, and lean mass. 5. Re-measure every two to four weeks, same spots, same conditions, morning before eating. Whichever method you pick, stay with it. Switching methods midway makes the trend impossible to read. --- ## How the Math Works The Navy method applies the Hodgdon-Beckett logarithmic formulas. Men: 86.010 x log10(waist - neck) - 70.041 x log10(height) + 36.76, all in inches. Women: 163.205 x log10(waist + hip - neck) - 97.684 x log10(height) - 78.387. The quick estimate uses the Deurenberg equation: 1.20 x BMI + 0.23 x age - 10.8 x sex - 5.4, with sex counted as 1 for men and 0 for women. Age enters the formula because people carry more fat at the same BMI as they get older. BMI alone cannot see muscle, so a muscular lifter gets an estimate that overshoots. Fat mass and lean mass split your entered body weight by the resulting percentage. Categories follow the ACE chart: the average band sits at 18 to 24 percent for men and 25 to 31 percent for women, with essential fat at 2 to 5 percent for men and 10 to 13 for women. Tape-based estimates carry three to four points of error even with careful measurement, so treat the output as a tracking tool. A DEXA scan gives the medical-grade number. --- ## Frequently Asked Questions ### How accurate is the Navy tape method? Within three to four percentage points of a DEXA scan for most people, as long as the tape work is careful. Tape placement is the main source of error. Measure at the same spots, under the same conditions, and the method tracks change well even when the absolute number sits a little high or low. ### Where exactly do I put the tape? Men measure the waist horizontally at the navel and the neck just below the larynx. Women measure the waist at the narrowest point of the torso, the neck the same way, and the hips at the widest point. Keep the tape level and snug without digging in, and breathe out normally before reading it. ### Why does the quick estimate ask for my age? The formula behind it, published by Deurenberg and colleagues, found that people carry more fat at the same BMI as they get older. Age corrects for that drift. The estimate is still rough, because BMI alone cannot see muscle. A muscular lifter will read high on BMI and get a body fat estimate that overshoots. ### What is a healthy body fat percentage? The ACE chart puts the average band at 18 to 24 percent for men and 25 to 31 percent for women, with the fitness band a step below that. Essential fat, 2 to 5 percent for men and 10 to 13 for women, is the floor the body needs to function. Sitting at the essential floor year round comes with real hormonal costs, so treat it as a competition phase rather than a target. ### Which method should I use? The Navy method, if you own a tape measure. It tracks your actual shape instead of inferring fat from height and weight. The quick estimate works for a rough figure when you have nothing to measure with. Whichever you pick, stay with it. Switching methods midway makes the trend impossible to read. ### How often should I re-measure? Every two to four weeks. Fat loss for most people runs slow enough that weekly readings mostly show measurement noise. Measure in the morning before eating, at the same spots each time, and take each measurement twice to catch tape slips. --- ## Forge Handles the Training Side Forge is an AI personal trainer for iOS and Android. It builds an adaptive workout plan around your goals, schedule, and equipment, and adjusts as you log your sessions. You get 4 trainer personalities, a 24/7 trainer chat, and full workout logging. Pricing: $19.99/month, $59.99/year, or $6.99/week. All plans include a 7-day free trial. - **App Store:** https://apps.apple.com/us/app/forge-ai-personal-trainer/id6758403402 - **Google Play:** https://play.google.com/store/apps/details?id=com.tendylabs.forge - **Website:** https://forgetrainer.ai --- *https://forgetrainer.ai/tools/body-fat-calculator*