Supplement calculators: check your dose, the upper limit and the cost
Five calculators answer the questions that come up with a bottle in your hand: is this too much zinc, how many fish oil capsules should I take, do I need vitamin D this month? Three more tools convert IU, check which supplements mix and explain VAT and duty.
Vitamin quiz: which vitamins do I need?
Which vitamins do you need? A free 9-question vitamin quiz based on NHS, NICE and NIH advice: what to take, what to test first and when to see your GP instead.
22 min readVitamin compatibility chart
Vitamin compatibility chart: vitamins that should not be taken together, gaps for iron, calcium and levothyroxine, and can you take magnesium and zinc together?
8 min readZinc calculator: food plus supplement
How much zinc per day do adults need, and how much zinc is too much? This calculator adds food zinc to your supplement and checks the total against UK and EU limits.
7 min readIU to mcg and mg converter
Convert IU to mcg or mg: vitamin D (1 mcg = 40 IU), vitamin A and beta-carotene (1 IU = 0.3 mcg RAE), vitamin E (1 IU = 0.67 or 0.45 mg). Reference table and FAQ.
9 min readHow many fish oil capsules a day
How much omega-3 per day do you need, and how many fish oil capsules a day is that? Enter EPA and DHA from your label to get softgels for 250 mg or 500 mg.
7 min readB6 checker for your B complex
Adults need under 2 mg of B6 a day, yet B complexes often hold 50–100 mg. Check your dose in a minute and see how much B6 is too much and the signs of B6 toxicity.
6 min readVitamin D sunlight calendar
When can you get vitamin D from sunlight in the UK? See month by month when the sun in London, Edinburgh, Dublin and other cities is too low for skin to make it.
9 min readiHerb VAT and customs charges checker
Will you pay VAT or import duty on iHerb? UK customs charges and the £135 rule, EU IOSS VAT and the €3 duty, and prepaid fees in Switzerland, in one checker.
7 min readSupplement cost per day calculator
Work out supplement cost per serving and per day in seconds. Compare bottles with different capsule counts and serving sizes, with a worked example and common traps.
6 min read
Which calculator do you need?
| Tool | What it works out | When to use it |
|---|---|---|
| Zinc intake calculator | Zinc from food plus your supplement, against the 25 mg and 40 mg limits | You’ve bought a 30–50 mg bottle or take zinc on top of a multivitamin |
| Omega-3 dose calculator | Softgels a day for 250 mg and 500 mg of EPA+DHA | The bottle says “1,000 mg” and you can’t tell how much omega-3 that is |
| B6 checker | Daily B6 from supplements against EFSA’s 12 mg and UK advice of 10 mg | You’ve taken a B complex or magnesium with B6 for more than a month |
| Vitamin D sunlight calendar | Months when the sun in your city is too low | You’re deciding when to start your autumn vitamin D |
| Cost per day | The cost of one day and one month of use | You’re comparing bottles of different sizes and serving sizes |
| IU to mcg converter | International units into mcg and mg | The dose on the bottle is in IU |
| Vitamin compatibility chart | Which supplements to take at different times | You take several supplements at once |
| iHerb VAT and duty | VAT and import charges on a UK or EU order | You want to know the final cost before you pay |
What do you want to check?
Pick a question to see which tool to open and what you'll need
Your question
The tool for it
Vitamin check
Up to 9 questions about your diet, sunlight, medicines and goals. It shows what NHS, NICE and NIH ODS advice says for people like you, which tests to ask about and what to watch out for.
You'll need: about 2 minutes and a few answers about you
The tool for it
Zinc intake calculator
Adds the zinc from your food to the zinc in your capsule and compares the total with EFSA's 25 mg and the US 40 mg limits. Handy if your bottle says 30 or 50 mg.
You'll need: zinc per capsule and your type of diet
The tool for it
Omega-3 dose calculator
Uses the EPA and DHA on your label to work out how many softgels give 250 mg and 500 mg a day. The "1,000 mg" on the front won't tell you.
You'll need: the EPA, DHA and serving size lines from the Supplement Facts panel
The tool for it
Vitamin B6 checker
Multiplies the B6 per capsule by capsules a day and compares it with EFSA's 12 mg limit and UK advice of 10 mg from supplements. A "B-50" usually has 50 mg.
You'll need: the Vitamin B6 line on the label and how many capsules you take
Numbness or tingling in your hands or feet while taking B6 means stop and see your GP.
The tool for it
Vitamin D sunlight calendar
Shows, month by month, when the midday sun in your city is too low for your skin to make much vitamin D.
You'll need: just your city
The tool for it
Supplement cost per day
Turns the bottle price into the cost of one day and one month at your dose. It's the fair way to compare bottles of different sizes and serving sizes.
You'll need: bottle price, capsules per bottle and capsules a day
The tool for it
IU to mcg converter
Turns international units into micrograms or milligrams. Each vitamin has its own factor: for vitamin D, 1 mcg = 40 IU; A and E differ.
You'll need: the vitamin and the IU figure on the label
The tool for it
Vitamin compatibility chart
Shows which pairs are fine together and which to space out, such as iron and calcium or minerals and levothyroxine. Also add up the zinc, B6, vitamin D and vitamin A in every bottle: limits apply to the total.
You'll need: a list of everything you take
The tool for it
iHerb VAT and duty checker
Explains how VAT and customs duty work on an iHerb order to the UK, the EU, Switzerland or Norway, including the £135 and €150 thresholds and the EU's €3 duty.
You'll need: just your delivery country
How to use the calculators
Keep the bottle in front of you: every figure comes from the back, not from the big number on the front.
- Find the Supplement Facts panel (on EU and UK products, the nutrition table).
- Check the serving size: every figure is per serving, and a serving is often 2–4 capsules.
- Enter the amount of the nutrient itself (elemental zinc, EPA and DHA), not the compound weight from the product name.
- Add up every source: multivitamin, separate supplement, powders and effervescent tablets.
- Read the result alongside the guide to that nutrient, which covers forms, who should avoid it and interactions.
Our guide on how to read a supplement label explains each line.
Where the numbers come from
Daily needs and limits come from the NHS, EFSA (the European Food Safety Authority) and NIH ODS (the US Office of Dietary Supplements). An upper limit is the highest daily intake that’s unlikely to harm healthy adults over the long term. You don’t aim for it: it’s a ceiling, not a “best dose”.
The limits the calculators use:
- zinc – 25 mg a day (EFSA, all sources); the NHS says no more than 25 mg a day from supplements; 40 mg in the US;
- vitamin B6 – 12 mg a day (EFSA, 2023); UK advice is no more than 10 mg a day from supplements; 100 mg in the US;
- vitamin D – 100 mcg (4,000 IU) a day for adults, according to the NHS;
- EPA and DHA – up to 5 g a day from supplements, which EFSA considers safe.
What the calculators can’t do
- diagnose anything or replace a blood test;
- set a treatment dose for a deficiency, which is your GP’s job;
- allow for pregnancy, kidney or liver disease, or medicines;
- use your exact diet: food zinc, for example, is an estimate of about 8–12 mg.
If you’re pregnant or breastfeeding, choosing supplements for a child, or taking blood thinners, epilepsy medicines or antibiotics, ask your GP or a pharmacist rather than a calculator. The same goes for numbness, tingling or nausea while taking supplements.
Can you rely on the calculator's answer?
Tick the statements that are true for you. If you can't tick one, talk to your GP or pharmacist first
0 / 6 ticked
All true – the calculators were built for exactly your situation. Check the result against the nutrient guide, then choose a bottle.
How to choose on iHerb with these tools
- Start with the nutrient guide, such as how much vitamin D3 to take or how to choose fish oil.
- Check the dose in the right calculator and make sure your total from all products stays under the limit.
- Compare two or three bottles by cost per day at the same dose of the active ingredient.
- Before you pay, check how VAT and delivery work for your country in our guide to ordering from iHerb.
FAQ
Can a calculator tell me a treatment dose?
No. The calculators compare your intake with the daily need and the upper limit for healthy adults. If a blood test shows a deficiency, your GP sets the treatment dose, which may be above the limit for a short time.
Why are EFSA limits lower than US ones?
Agencies weigh the same studies differently and build in different safety margins. For vitamin B6, EFSA cut its adult limit to 12 mg in 2023, while the US limit is still 100 mg. For long-term use we go with the more cautious European figure, which is close to UK advice.
Do I need to count vitamins from food?
For zinc and B6, yes: EFSA limits cover all sources, and the zinc calculator adds an estimate for your diet. The omega-3 calculator counts capsules only; eating oily fish lowers the amount you need from a supplement.
Is the information I type in saved?
No. The maths runs in your browser and nothing is sent or kept.