Fitness tools that show their working
Your phone records beautifully and interprets almost nothing. These do the parts that need a model — separating a real weight change from two kilos of water, working out what you actually burn from your own history rather than a formula, turning two hard efforts into the pace you can hold — and every figure comes with how sure it is. Nothing is uploaded, there is no account, and there is nothing to subscribe to.
None of this is medical advice. These are calculators: they work out what your own numbers imply and say how sure that is. They know nothing about your health, they are written for adults, and they are not a substitute for a clinician — particularly if you are pregnant, managing a condition, or have any history of disordered eating.
The tools
Three, doing the three things the arithmetic can genuinely help with.
What a health app records, and what it never tells you
Apple Health and Samsung Health are excellent diaries. What they do not do is any of the interpretation, and that is where every decision actually lives.
| You want to know | What the app shows | What is here |
|---|---|---|
| Am I actually losing weight? | A sawtooth chart of every reading | The trend under the noise, and whether the change is bigger than the scatter |
| What do I actually burn? | A formula times an activity guess | Your own intake and weight change, run backwards — no formula |
| Is this a plateau? | Nothing | A plateau and "not enough data yet" told apart, with the reason |
| Am I getting stronger? | Workout minutes | Estimated max across five formulas, trended, with the disagreement shown |
| What pace can I hold? | A VO2 max with no working | VDOT and critical speed from efforts you actually did |
| Are my zones right? | Zones from 220 minus your age | Zones from a threshold you can measure in one session |
| Have I ramped up too fast? | Rings and streaks | This week against the last month, with the honest caveat attached |
And the part no subscription offers: none of it leaves your device. There is no account to make, no export to hand over, and nothing to cancel.
What these will not do
They will not diagnose anything. A calculator cannot know about a thyroid, a medication, a pregnancy or a condition, and every number here assumes an ordinary healthy adult. Where a figure falls outside a healthy range the tools say so and stop rather than optimising.
They will not invent your data. Nothing estimates your body fat from your age, or your maximum heart rate from a formula, and then builds on it silently. Where a measurement is needed, it is asked for; where a figure is a convention rather than physics — 7,700 kcal to a kilogram, the 1.06 in Riegel's rule — it is named as one.
They will not pretend to be certain. Every estimate here carries the range its data actually supports, and several will refuse to answer until there is enough of it. A single number from five noisy weigh-ins is a lie told precisely, and the industry is full of them.
If food or weight is difficult
Turning eating into arithmetic makes things worse for some people, and daily weighing is specifically discouraged in several treatment approaches. If that is you, these are not the right tools, and that is not a failing on your part. In the UK, Beat's helpline is 0808 801 0677; elsewhere, your GP is the right first call.