Health
How to Estimate Dehydration Risk in a Session
Learn the simple math behind session dehydration, including the 2% body-weight threshold, and use the planner to check your numbers.
Aug 13, 2026 · 5 min read
Category
Health
Fitness, nutrition, and wellbeing backed by what works.
27 tools · 29 articles
Free Health tools
Dehydration Risk Session Planner
Estimate how much body weight you could lose in a session and whether it passes the 2% dehydration threshold.
Open the tool →Blood Pressure Category Calculator
Where a reading falls on the American Heart Association's categories - and which of your two numbers is the one deciding it.
Open the tool →Body Surface Area Calculator
Mosteller and Du Bois side by side, because they disagree - and the gap is the reason this is a clinician's number rather than yours.
Open the tool →Cycling Power to Weight Calculator
Watts per kilo, the climbing speed it implies, and what a kilo off the bike is actually worth in seconds.
Open the tool →Daily Sodium Intake Calculator
Sodium and salt are not the same number - 5 g of salt is 2,000 mg of sodium. Confusing them is a two-and-a-half-fold error.
Open the tool →Daily Fibre Intake Calculator
The 25 g and 38 g figures are not about being a woman or a man - they are 14 g per 1,000 calories at two assumed appetites.
Open the tool →Sweat Rate and Hydration Calculator
Weigh yourself before and after a session to find your real sweat rate - which is the CEILING on what to drink, not a target to hit.
Open the tool →Sleep Debt Calculator
Add up a week honestly. The arithmetic is easy - it is the idea that a weekend lie-in repays it that does not survive contact with the evidence.
Open the tool →Critical Swim Speed Calculator
Your threshold pace from a 400 and a 200 time trial - and why it is slower than your 400 pace, which is the point people get wrong.
Open the tool →Training Load Ratio Calculator
How this week compares with your recent average - worked both ways, so you can see the mathematical coupling the popular version hides.
Open the tool →VO2 Max Estimator (Cooper Test)
Run as far as you can in 12 minutes. One subtraction, one division - and about ten percent of error, which the tool reports as a range.
Open the tool →Waist to Height Ratio Calculator
Keep your waist to less than half your height. The ratio, the bands, and the actual waist measurement each band means for you.
Open the tool →Walking Calorie Calculator
The figure your tracker shows includes what you would have burned sitting down. The walk itself added considerably less.
Open the tool →Sleep Cycle Calculator
Five cycles is 7h30m at the average — but anywhere from 5h50m to 10h once you use the real 70-120 minute range. The 7-hour floor is the sturdier number.
Open the tool →BMI Calculator
Your BMI in either unit system, and the weight range it corresponds to for your height - plus an honest account of what the number cannot tell you.
Open the tool →Body Fat Calculator (Navy Method)
The Hodgdon-Beckett circumference formula, in INCHES - which matters, because feeding the same constants centimetres reports 26.7% for a body that is 20.1%.
Open the tool →Calorie Deficit Timeline Calculator
How long a given deficit takes - built on the 3,500-calorie rule, which researchers have shown overestimates real weight loss. The tool says so.
Open the tool →Heart Rate Zone Calculator
Training zones from the Tanaka formula rather than 220 minus age, using your resting heart rate - so 'easy' comes out as a number you can actually hold.
Open the tool →Ideal Weight Calculator (Devine & Hamwi)
Two published formulas, side by side, disagreeing by pounds - and both were built to dose medication rather than to tell you what to weigh.
Open the tool →Macro Split Calculator
Percentages into grams - and a check that your three add to 100, which is where most macro plans quietly fail.
Open the tool →One-Rep Max Calculator
Estimate your 1RM from a set you actually did, using both Epley and Brzycki side by side - two published formulas that cross at exactly ten reps.
Open the tool →Protein Intake Calculator
The 0.8 g/kg RDA is a minimum to avoid deficiency, not a target. If you train, the published range is roughly double it.
Open the tool →Race Time Predictor
Predict a race time from one you have already run, using Riegel's formula - and see how much your pace is expected to slow over the longer distance.
Open the tool →Running Pace Calculator
Turn a distance and a finish time into your pace per mile and per kilometre, with the speed in both units - the numbers you need to pace the next one.
Open the tool →Steps to Miles Calculator
Converts steps using YOUR height rather than the '2,000 steps per mile' rule - which quietly describes someone 6 ft 4 in tall.
Open the tool →TDEE Calculator
What your body burns in a day, using the Mifflin-St Jeor equation - split into the part that is just staying alive and the part your activity adds.
Open the tool →Daily Water Intake Calculator
The 8x8 rule is 64 oz. The actual reference figure is 125 oz for men and 91 for women - and unlike 8x8, it counts food and every other drink.
Open the tool →Health articles
Health
Can You Actually Catch Up on Sleep?
Five weekday nights lose 6.5 hours; two weekend nights recover 1.5. That's arithmetic before it's ever physiology.
Aug 3, 2026 · 3 min read
Health
How Much Fibre Do You Actually Need?
25 g and 38 g aren't about being a woman or a man. They're 14 g per 1,000 calories at two assumed appetites.
Aug 3, 2026 · 3 min read
Health
How Much Should You Actually Drink During Exercise?
Your sweat rate is a ceiling, not a target. Hyponatremia is almost always caused by drinking too much — and it has killed diligent athletes.
Aug 3, 2026 · 3 min read
Health
How to Find Your Swim Threshold Pace
Two maximal swims and one subtraction. And your threshold is slower than your 400 pace — which is the bit people get wrong.
Aug 3, 2026 · 4 min read
Health
Sodium and Salt Are Not the Same Number
5 g of salt is 2,000 mg of sodium. Labels differ by country, and reading one as the other is a 2.5-fold error.
Aug 3, 2026 · 4 min read
Health
The Acute:Chronic Workload Ratio Is Not What You Were Told
Double your training week and the conventional ratio reports 1.64 instead of 2.09 — because this week is inside its own denominator.
Aug 3, 2026 · 4 min read
Health
What a 12-Minute Run Tells You About Your Fitness
One subtraction and one division gives your VO₂ max. It also carries about ±10% of error, so the honest answer is a range.
Aug 3, 2026 · 4 min read
Health
What Does Watts Per Kilo Actually Buy You?
It decides climbs and does almost nothing on the flat. And a kilogram off the bike is worth about 22 seconds up a 500-metre climb.
Aug 3, 2026 · 4 min read
Health
What Is Body Surface Area and Why Does It Matter?
It sets chemotherapy doses and indexes kidney function — and two standard formulas give two different answers for the same person.
Aug 3, 2026 · 4 min read
Health
What the Two Blood Pressure Numbers Mean
118 over 85 is Stage 1 hypertension. Most people read the first number and stop - and that is the number that isn't deciding it.
Aug 3, 2026 · 5 min read
Health
Why Waist to Height Beats BMI
Keep your waist to less than half your height. One rule, no chart — and it sees something BMI structurally cannot.
Aug 3, 2026 · 4 min read
Health
Your Walk Burned Fewer Calories Than Your Tracker Says
A 5 km walk shows 259 on the display. It added 186 — the other 73 you'd have burned sitting down.
Aug 3, 2026 · 4 min read
Health
How Many Sleep Cycles Do I Need?
Five 90-minute cycles is 7h30m. But five real cycles is anywhere from 5h50m to 10h — which is why the 7-hour total is the number worth planning around.
Aug 1, 2026 · 5 min read
Health
How Long Does It Take to Lose 20 Pounds?
The 3,500-calorie rule says 20 weeks at a 500/day deficit. Researchers have shown it overestimates real loss - and gets worse the further out you look.
Jul 31, 2026 · 4 min read
Health
How Many Calories Do I Burn in a Day?
The Mifflin-St Jeor equation, and the honest part: most of your burn is just staying alive, and the input you'll get wrong is the activity level.
Jul 31, 2026 · 3 min read
Health
How Many Steps Are in a Mile?
Not 2,000 — that figure quietly describes someone 6 ft 4 in tall. For most people it's between 2,200 and 2,400.
Jul 31, 2026 · 4 min read
Health
How Much Protein Do I Need?
The 0.8 g/kg figure everyone quotes is an RDA — the amount that prevents deficiency. If you train, the published range is roughly double it.
Jul 31, 2026 · 3 min read
Health
How Much Water Should I Drink?
The 8×8 rule is 64 oz. The reference figure is 125 for men and 91 for women — and unlike 8×8, it counts every drink and the water in your food.
Jul 31, 2026 · 3 min read
Health
How the Navy Body Fat Formula Works
Its constants are for INCHES. Feed the same equation centimetres and it reports 26.7% for a body that is 20.1% — and nothing looks broken.
Jul 31, 2026 · 4 min read
Health
How to Calculate Your Macros
Fat is 9 calories a gram, not 4 — so a diet that's 25% fat by calories is only 13% fat by weight. And nothing checks that your percentages add to 100.
Jul 31, 2026 · 4 min read
Health
How to Estimate Your One-Rep Max
Two published formulas, side by side — and the thing nobody mentions: they cross at exactly ten reps, which changes how you should read the gap between them.
Jul 31, 2026 · 4 min read
Health
How to Predict Your Race Time
Riegel's formula turns one race into a prediction for another. It's reliable over small jumps — and it quietly assumes you've actually trained for the distance.
Jul 31, 2026 · 4 min read
Health
What Are My Heart Rate Zones?
220 minus age has no research behind it, and it is wrong in opposite directions either side of 40. Here is a better formula, and what easy means in beats.
Jul 31, 2026 · 4 min read
Health
What Does My BMI Actually Mean?
The number is easy to get and easy to over-read. Here's what it measures, what it provably can't, and the more useful figure sitting underneath it.
Jul 31, 2026 · 4 min read
Health
What Is My Ideal Weight?
These formulas were built to dose medication, not to tell you what to weigh — and for a 6'6" man two of them disagree by 12.5 pounds.
Jul 31, 2026 · 4 min read
Health
What Is My Running Pace?
Turn a distance and a finish time into minutes per mile and per kilometre. Then the more useful question: what that average is actually hiding.
Jul 31, 2026 · 4 min read
Health
Meal Delivery in 2026: Health Trend or Hype?
Hyper-personalized meal delivery is getting attention in 2026. Here’s what it can actually do for your health — and what to ignore.
Jul 7, 2026 · 4 min read
Health
Longevity Without the Biohacking: The Boring Basics That Actually Work
Forget the $5,000 supplement stacks. The habits that actually extend a healthy life are unglamorous, mostly free, and well-proven.
Jun 17, 2026 · 2 min read