Calculator
Magnesium: cost per effective dose
Magnesium supplements should be compared on elemental magnesium, the actual magnesium content, not the compound weight on the front of the tub: magnesium oxide is about 60 percent magnesium, citrate about 16 percent and glycinate about 14 percent. This calculator converts any product to pence per 100 mg of elemental magnesium and prices it against the NHS 270 mg daily figure for women. Information only, not medical advice.
How the calculation works
We multiply the labelled milligrams by the elemental fraction of the form you select (calculated from molecular weights: oxide 60.3 percent, citrate 16.2 percent, glycinate 14.1 percent, malate 15.5 percent, chloride 12 percent, taurate 8.9 percent), then divide the cost per serving by that elemental amount. The headline number is pence per 100 mg of elemental magnesium, plus the cost of an NHS reference day (270 mg for women aged 19 to 64). If your label already states elemental magnesium, choose that option and no adjustment is applied. Elemental fractions are calculated from molecular weights. Real products vary; if the label states elemental magnesium, use that figure.
Reference figures: 270 mg a day (women 19 to 64) and the 400 mg supplement caution from NHS, Others: vitamins and minerals (magnesium), as of 2026-06-11. Elemental fractions are arithmetic from molecular weights.
Information only, not medical advice. This tool is educational and is not a substitute for a registered clinician. It does not diagnose anything and does not recommend that you take any supplement. Always read product labels and speak to your GP or pharmacist before starting, stopping or combining supplements.
Read more
See our magnesium guide, the glycinate vs citrate comparison, when to take magnesium and the best magnesium for women roundup. More calculators on the cost per effective dose hub.
Frequently asked questions
Why does the magnesium form matter for price?
Because the label weight is mostly not magnesium. Magnesium oxide is about 60 percent elemental magnesium, but citrate is roughly 16 percent and glycinate roughly 14 percent, so "500 mg magnesium glycinate" can contain around 70 mg of actual magnesium. Two products at the same pack price can differ several-fold in what you pay per milligram of magnesium.
How much magnesium do women need a day?
The NHS figure is 270 mg a day for women aged 19 to 64, and most people can get that from food. The NHS also notes that taking 400 mg or less a day of magnesium in supplements is unlikely to cause harm. Whether you need a supplement at all is a conversation for your GP or pharmacist.
Does my label show elemental magnesium or compound weight?
UK labels are required to declare the elemental amount in the nutrition table (usually alongside a percent NRV), but marketing copy often quotes the bigger compound weight. If your label states elemental magnesium, choose the elemental option in the calculator; otherwise pick the form and we apply the molecular-weight fraction.
Is glycinate worth the premium over oxide?
Glycinate and citrate are commonly chosen for gentler digestion, and oxide is the cheapest per milligram but less well absorbed. The calculator shows you exactly what the premium is per milligram of elemental magnesium so you can decide if it is worth it. Our magnesium glycinate vs citrate comparison covers the evidence.
Editor, Her Vitals
Oliver leads Her Vitals's editorial coverage of women's life-stage health and supplements. He curates and reviews existing branded products across trying to conceive, pregnancy, postnatal, perimenopause, menopause and the senior years, weighing what the evidence supports against guidance from bodies such as EFSA, the NHS and NICE, and is clear that the content is information rather than medical advice.
Last reviewed: 11 June 2026