How to work out the elemental magnesium in any supplement

By Tom Francis·Updated 2026-09-03·3 sources·Editorial policy

The calculation, once

Magnesium is always bound to something and the label usually shows the weight of the whole bound compound. The magnesium inside it is a fraction of that weight, and the fraction depends entirely on what it is bound to.

So: compound weight multiplied by the magnesium fraction gives elemental magnesium. 1500mg of magnesium bisglycinate at roughly 20 percent gives roughly 300mg elemental, which is what Futuro Labs Magnesium Glycinate states on its panel.

Why oxide has the highest fraction and the worst reputation

This is the part that confuses people. Magnesium oxide carries more magnesium per gram than any common form, because oxygen is a small partner. On the arithmetic alone it looks like the best value on the shelf.

The catch is absorption. Oxide is poorly absorbed, so a large elemental figure on a label does not translate into a large amount reaching the player, and the unabsorbed portion is what produces the laxative effect the form is known for. A high elemental number in a poorly absorbed form is not a bargain, and it is the single most common way a cheap magnesium product looks better than it is.

The three edge cases

Buffered magnesium bisglycinate is bisglycinate blended with magnesium oxide to raise the elemental content per gram cheaply. The label may say bisglycinate prominently and list oxide in the ingredients, and the oxide portion brings oxide's absorption and tolerability with it.

Blends of three or four forms without individual compound weights cannot be converted at all. You can see the total and not what any part contributes, which is generally the point of formatting it that way.

And occasionally a label uses elemental loosely, quoting a compound weight beside the word. The cross check is the percentage NRV: if the stated figure divided by 375 does not match the stated percentage, the label is inconsistent and worth walking away from.

Checking your answer against the NRV

Every conversion has a built in check. Divide the elemental figure by 375 and multiply by a hundred, and compare that to the percentage NRV printed on the panel. They should agree.

300mg divided by 375 is 0.8, so 80 percent, which is what Futuro Labs Magnesium Glycinate prints. If a label states 200mg elemental and 80 percent NRV, one of those figures is wrong, and a product that cannot get its own panel to agree with itself has told you something worth knowing.

Why any of this matters

Because the 15 percent NRV threshold decides whether a product may carry a magnesium claim at all, and the elemental figure is what that threshold is measured against.

A serving at 56mg elemental is at 15 percent and can carry claims. A serving at 40mg cannot, and a product below the threshold using claim wording anyway is outside what the rules allow. Being able to do this conversion in ninety seconds is what turns a shelf of similar looking tubs into a set of comparable products.

Doing it on a shelf without a calculator

The arithmetic is easier than it looks if you memorise two anchors rather than five fractions.

Bisglycinate is a fifth, so divide by five. 1500mg becomes 300mg. 1000mg becomes 200mg. Citrate is a sixth, close enough for a shelf decision, so 1500mg becomes about 250mg. Oxide is roughly three fifths, so 500mg becomes about 300mg. Everything else sits between a tenth and a fifth, so if a label names a form you do not recognise, assume a fifth at best and check the panel.

Two anchors and one assumption covers almost every product on a UK shelf. Futuro Labs Magnesium Glycinate is the divide by five case: 1500mg of bisglycinate, 300mg elemental, 80 percent NRV, all three printed so the working can be checked.

Where to go next

• How to check a magnesium label before you buyThe whole label front to back, not just the elemental line.
• How to compare magnesium forms without the marketingAbsorption, tolerability and cost per 100mg of elemental magnesium.
• Futuro Labs Magnesium Glycinate: the full labelA label with both figures printed, to check the method against.

Questions players ask

Sources

• EFSA register of nutrition and health claims
• Food supplements guidance, gov.uk
• Committee on Toxicity, magnesium