Free calculators for the doses nobody agrees on
Every supplement label prints one number, every article prints another, and neither knows anything about you. These six free tools do the arithmetic properly — using your body weight, age, diet, and health conditions — and they tell you plainly when you do not need a supplement at all.
Find the right tool
Showing all 6 calculators.
"Is my 1,000 mg capsule really 1,000 mg of magnesium?"
No, and that gap is the whole problem. This tool works in elemental magnesium, the number that actually counts, and converts it into capsules of the form you are taking.
Anchored to: NIH Office of Dietary Supplements — RDA by age and sex, 350 mg supplemental upper limit.
Open the calculator →"Should I buy the 1,000, 2,000, or 5,000 IU bottle?"
Answers it from your sun exposure, skin tone, body weight, age, and whether a blood test came back low. The 5,000 IU option stays locked unless you have actually been tested.
Anchored to: NIH ODS — RDA 600–800 IU, 4,000 IU upper limit, 1 mcg = 40 IU.
Open the calculator →"How many grams, and do I actually need to load?"
Scales the dose to your body weight with a slider, builds the optional loading protocol, and shows why a 23 g loading day is four scoops of powder but twenty-three capsules.
Anchored to: ISSN Position Stand 2017 and the 0.3 g/kg loading protocol.
Open the calculator →"How many grams of protein do I really need?"
Sets a daily target from your weight, build, age, activity and goal, splits it across your meals, then shows how much your current eating already covers — and whether a powder is pointless for you.
Anchored to: Harvard Health RDA, Stanford Medicine 2026, Mayo Clinic per-meal guidance.
Open the calculator →"What time should I actually take it?"
The only one of these that answers with a clock time, because with melatonin the hour matters more than the milligrams. Handles jet lag by direction, night shifts, and a body clock stuck too late.
Anchored to: Johns Hopkins Medicine, Mayo Clinic, and the product's own label warnings.
Open the calculator →"How much, and capsule, chewable, or powder?"
Two answers in one: a milligram target from the reference intakes and your diet, then a match to the format you will still be taking in a month. Adjusts hard for kidney stones and iron overload.
Anchored to: NIH ODS — 90/75 mg RDA, +35 mg for smokers, 2,000 mg upper limit.
Open the calculator →Why these exist: supplement dosing goes wrong in three predictable ways — people take a compound weight and think it is the elemental dose, they take a number meant for an average adult when they are not one, and they take more on the assumption that more works better. Each of these tools is built to catch one of those.
How these calculators are built
Anyone can put a form on a page and multiply your weight by a number. The parts that take actual work are the sourcing and the safety logic, so here is exactly what is behind them.
Which one do you need?
If you are here with a specific question rather than a specific supplement, this is the quickest route in.
| If you are asking | Start here | What you get back |
|---|---|---|
| "I cannot fall asleep" | Melatonin | A clock time and a starting dose |
| "I wake up sore and stiff, or get cramps" | Magnesium | Elemental mg, and capsules of your form |
| "I want to get stronger" | Creatine | Daily grams, plus loading if you want it |
| "Am I eating enough protein?" | Protein | A daily target and your current shortfall |
| "I am indoors all winter" | Vitamin D | The right IU strength for your situation |
| "I catch every cold going" | Vitamin C | A mg target and the format that fits you |
| "I am flying to another continent" | Melatonin | A jet lag schedule by direction and zones |
| "My skin is showing my age" | Vitamin C | Collagen cofactor dosing and format |
Frequently Asked Questions
Are these calculators free? +
Yes, completely. There is no sign-up, no email wall, and nothing is stored. Every calculation runs in your own browser, so nothing you enter about your health leaves your device.
Do you save the health information I enter? +
No. The tools are plain JavaScript running locally in your browser. Your weight, age, and health conditions are never sent to a server, never logged, and disappear the moment you close the tab.
Where do the doses come from? +
Public reference intakes and position statements: the NIH Office of Dietary Supplements for the vitamins and minerals, the ISSN position stand for creatine, and Harvard Health, Mayo Clinic and Johns Hopkins for protein and melatonin. Each tool links its sources inline.
Can I use these instead of seeing a doctor? +
No. They give general educational estimates for healthy adults and cannot account for your medical history, medications, or lab results. Several of them will refuse to give you a number and tell you to see a clinician, which is the point.
Why does a calculator sometimes refuse to give me a dose? +
Because for some conditions the honest answer is not a number. Kidney disease changes magnesium and protein handling. Melatonin is ruled out by its own label for several conditions. In those cases the tool hides the dose and the product link rather than guessing.
Do the tools work on a phone? +
Yes. Every tool is built mobile-first with large tap targets, and the results panel moves below the questions on a narrow screen so you can see your answer update as you go.
Which calculator should I start with? +
Start from the problem, not the supplement. If sleep is the issue, the melatonin tool. If it is training, creatine or protein. If you are simply not sure whether you need anything, the protein and vitamin C tools are both willing to tell you that you do not.
Will you add more calculators? +
Yes. A supplement and drug interaction checker, a tincture to capsule converter, and a stack planner that flags timing conflicts are the next three. If there is one you want, tell us what question it should answer.
Medical Disclaimer
These calculators provide general educational estimates for healthy adults only. They are not medical advice, a diagnosis, or a treatment plan, and they cannot account for your full medical history, medications, allergies, or laboratory results.
Always talk to a doctor or pharmacist before starting or changing any supplement, and in particular if you:
- have kidney disease, liver disease, diabetes, a thyroid or endocrine condition, or any chronic illness;
- are pregnant or breastfeeding;
- take prescription medication of any kind;
- are choosing a dose for anyone under 18 years of age.
Never exceed the upper limits shown in each tool without medical supervision, and stop taking any supplement and seek medical care if you develop unexpected symptoms.
These statements have not been evaluated by the FDA. These products are not intended to diagnose, treat, cure, or prevent any disease.