Best Free BMI Calculator Online (2026) — Accurate, Private, No Signup

Updated April 20, 2026 · 8 min read

Reviewed by the EverydayTools Editorial Team

Quick answer: The NIH BMI Calculator and EverydayTools BMI Calculator are the most accurate free options using the standard WHO formula. The CDC calculator is best for children. All use the same calculation — the differences are in privacy, extra features (healthy weight range, unit conversion), and UI quality.

Every major health website offers a free BMI calculator. Most of them run the exact same formula. The real differences are in privacy (does it send your data to a server?), unit support (metric vs imperial), additional outputs (ideal weight range, BMI category details), and interface quality. We compared the most popular options so you can pick the right one for your needs.

Top Free BMI Calculators Compared (2026)

CalculatorPrivacyMetric + ImperialHealthy Weight RangeChildrenSignup
EverydayTools✓ Browser-only✓ Both✓Adults onlyNo
NIH (NHLBI)Server-side✓ Both✓Adults onlyNo
CDCServer-side✓ BothLimited✓ Separate toolNo
HealthlineAd-heavy, tracking✓ Both✓NoNo
Calculator.netServer-side✓ Both✓✓No

Detailed Reviews

1. EverydayTools BMI Calculator — Best for Privacy + Full Feature Set

The EverydayTools BMI Calculator runs entirely in your browser using JavaScript. No height, weight, or age data is sent to any server. It supports metric (kg/cm) and imperial (lbs/ft/in) with automatic unit conversion when you switch. Results include BMI value, WHO category, and healthy weight range for your height.

Best for: Anyone who wants a clean, fast BMI calculation without ads tracking their health data. Ideal for apps that embed a BMI tool, and for users who prefer health data staying on their device.

Limitations: Adults only (18+). Does not include body fat percentage or waist-to-height ratio measurements.

2. NIH / NHLBI BMI Calculator — Most Authoritative Source

The National Heart, Lung, and Blood Institute (NHLBI) BMI calculator is the gold standard for medical and insurance purposes. When a doctor, employer, or insurer refers to BMI, they're using the same WHO formula this tool implements. The interface is minimal but trusted.

Best for: Medical documentation, insurance applications, and any context where you need a result from an officially recognized government health agency.

Limitations: Outdated UI. No healthy weight range shown. No metric support (imperial only). Data is sent to NIH servers.

3. CDC BMI Calculator — Best for Children and Teens

The CDC BMI Calculator is unique in offering a separate, age-appropriate tool for children and teens (ages 2–19). Unlike adult BMI, children's BMI is interpreted using sex- and age-specific percentile charts. The CDC tool handles this automatically and explains results in context of the growth charts.

Best for: Parents assessing a child's weight, pediatric healthcare contexts, and school health screenings. The CDC child BMI tool is the US standard for ages 2–19.

4. Calculator.net — Most Feature-Rich

Calculator.net offers an unusually comprehensive BMI tool that includes adult BMI, children's BMI by age, ideal weight calculation, BMI prime (ratio of BMI to upper normal), and ponderal index. It also shows BMI tables and charts. If you want the most data from a single tool, Calculator.net delivers more outputs than any other free option.

Best for: Fitness professionals and health coaches who want comprehensive data outputs including ideal weight ranges, BMI tables, and multiple body composition metrics in one place.

Do All BMI Calculators Give the Same Result?

Yes — if they implement the standard WHO formula correctly. BMI = weight (kg) ÷ height (m)². A 70 kg person who is 1.75 m tall has a BMI of exactly 22.86 in every calculator that uses this formula. The only variation comes from rounding (some show 1 decimal, others 2) and how they handle imperial-to-metric conversion.

If you ever get a different result from two calculators, check:

  1. Whether the calculator is using the same unit (kg vs lbs, cm vs inches)
  2. Whether height input uses feet+inches separately or decimal feet
  3. Rounding differences in the final display

Verdict

For adults who want a fast, private BMI calculation: use EverydayTools. For official medical documentation: use NIH. For children: use CDC. For the most comprehensive data output: use Calculator.net.

Remember: BMI is a screening tool, not a diagnosis. A BMI result outside the normal range should prompt a conversation with a healthcare provider — not a self-diagnosis. For a more complete picture of body composition, combine BMI with waist circumference and, if possible, body fat percentage measurements.

More Free Health Calculators