Built on current US clinical guidelines
Understand your health numbers
Free calculators with plain-English explanations — what each result means, where it comes from, and where it stops being useful.
An information and calculation service — not a medical device, not a healthcare provider, and no substitute for a clinician who can examine you. Medical disclaimer
Health tools
Popular calculators
Each tool shows the formula it uses, the clinical guideline behind its categories, and the limitations that apply.
Waist-to-Height
General Wellness
Divide your waist by your height and read the result against the bands NICE publishes.
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Waist-to-Hip
General Wellness
Compare your waist and hip measurements against the cut-points in the WHO expert consultation.
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Body Fat %
Fitness
Estimate body fat from tape measurements with the Hodgdon–Beckett circumference equations.
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Body Surface Area
General Wellness
Calculate body surface area with four published equations, and see how far apart they land.
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Ideal Body Weight
General Wellness
Compare the Devine, Robinson, Miller and Hamwi equations — and see how much they disagree.
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Creatinine Clearance
Kidney Health
Estimate creatinine clearance with Cockcroft-Gault, and see why it differs from eGFR.
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Cholesterol Converter
Blood Tests
Convert a whole lipid panel between mg/dL and mmol/L, with the correct factor for each value.
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Water Intake
Nutrition
See the US and European reference values for water side by side, rather than one invented number.
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Health topics
Explore by topic
Organized hubs that bring together the calculators, guides, and frequently asked questions for each area of health — from heart health to metabolic screening.
Heart Health
Blood pressure, cholesterol and the numbers behind cardiovascular risk.
7 tools · 14 guides
Health library
In-depth guides
Plain-English explanations of the measurements that appear most often on a lab report — what each number means, how it is measured, and what to discuss with your doctor.
Diabetes & Metabolic HealthWhat Does HbA1c Mean?
HbA1c measures the percentage of your hemoglobin that has glucose attached to it, which reflects your average blood glucose over roughly the previous two to three months. It is…
9 min read
Diabetes & Metabolic HealthA1C vs Fasting Blood Sugar: What’s the Difference?
A fasting blood sugar test measures glucose at one moment after not eating; A1C reflects average glucose over two to three months. They answer different questions, they can disa…
7 min read
Heart HealthUnderstanding Blood Pressure Numbers
The top number is the pressure as your heart contracts, the bottom number is the pressure between beats. In the United States the categories come from the 2025 AHA/ACC guideline…
13 min read
Heart HealthLDL, HDL, Triglycerides and Total Cholesterol Explained
Cholesterol travels in the blood inside particles. LDL carries it toward artery walls, HDL carries it away, and triglycerides are a separate fat in their own particles. The 2026…
13 min read
General WellnessWhat Is BMI and What Does It Actually Tell You?
BMI is weight divided by height squared — a population screening tool invented in the 1830s, not a measure of an individual’s health. It cannot distinguish muscle from fat, says…
10 min read
Kidney HealthWhat Is eGFR? Understanding Kidney Function
eGFR estimates how much blood your kidneys filter each minute, calculated from creatinine, age and sex rather than measured directly. A single result does not establish kidney d…
9 min read
Health Explorer
Learn one measurement at a time
Guided learning journeys that pair each calculator with its explanation and a short quiz. Progress is saved in your browser — no account required.
Understand Your A1C
Work through what HbA1c measures, how it relates to day-to-day glucose, and where it can mislead.
5 steps
Understand Blood Pressure
Learn what the two numbers mean, how guidelines differ, and what pulse pressure and MAP add.
5 steps
Understand Cholesterol
Work through the lipid panel: what each fraction does, what non-HDL adds, and how LDL is calculated.
4 steps
Understand Kidney Numbers
Learn what creatinine and eGFR measure, why the equation changed, and what one result cannot establish.
4 steps
Why Gravis Labs
Built to be useful, not just popular
A health number is only useful if you know how it was produced and where it breaks down. Transparency is the foundation of everything we build.
Instant and simple
Enter a value, get a clear answer. No accounts, no sign-ups, no clutter — just the calculator and the explanation behind it.
Genuinely educational
Every calculator comes with the formula it uses, a plain-English explanation of what the result means, and an honest account of where it stops being useful.
Clinically current
Thresholds are drawn from guidelines in force in the United States — including the 2025 AHA/ACC blood pressure guideline, the 2026 ACC/AHA cholesterol guideline, and the ADA’s 2026 Standards of Care — each named and cited.
Private by design
Every calculation runs entirely in your browser. The health values you enter are never transmitted to a server, never stored, and never included in analytics.