You’ve picked a form. That’s the easy bit, and the types guide walks you through it. But two bottles of “magnesium glycinate” sitting side by side can be very different products, and the label is built to make that hard to see. A few things separate a good one from a waste of money.
First, the number that matters: elemental magnesium
This is the most common way people overpay, so it’s worth repeating. The big number on the front of the bottle is usually the weight of the whole compound: magnesium plus whatever it’s bound to. What your body works with is the elemental magnesium, a smaller number in the fine print on the back. Two products can both say “1,000mg” on the front and deliver wildly different amounts of usable magnesium. Compare the elemental figure per serving, or you’re not comparing anything.
Here’s what 1,000mg of each compound actually contains:
Notice the trap. Oxide carries far more magnesium per gram than anything else, and it’s still the form I’d steer you away from, because you absorb so little of it. A big number on the front and a poor result are perfectly compatible.
How to check this yourself
You don’t have to take my word for any of the figures above. They’re arithmetic from the periodic table.
Take magnesium oxide, MgO. Magnesium’s atomic weight is 24.305 and oxygen’s is 15.999, so the compound weighs 40.304. Magnesium’s share is 24.305 ÷ 40.304, which is 60.3%.
Bisglycinate is Mg(C₂H₄NO₂)₂, so there are two glycine groups. Each carries two carbons, four hydrogens, one nitrogen and two oxygens, so doubling them gives four carbons, eight hydrogens, two nitrogens and four oxygens, plus the magnesium. That totals 172.4, of which magnesium is 24.305, or 14.1%.
Citrate is the fiddly one, Mg₃(C₆H₅O₇)₂, because there are three magnesium atoms to two citrate groups. Two citrates give twelve carbons, ten hydrogens and fourteen oxygens. The compound weighs 451.1 and the three magnesiums together weigh 72.9, giving 16.2%.
The hydrate caveat
Those figures are for the anhydrous compounds, the pure salt with no water attached. Several of these are commonly sold as hydrates, where water molecules are part of the crystal, and that water counts towards the weight without adding any magnesium.
Magnesium citrate is the clearest example. As the nonahydrate, which is a common commercial form, the elemental share drops from 16.2% to roughly 11 or 12%. So a product’s real figure can sit well below the pure-compound number.
This is precisely why the label wins over any calculation you do at home. Use the arithmetic to understand why the front-of-pack number misleads, then buy on the elemental figure the manufacturer prints on the back.
Check how many capsules make a “serving”
A label might promise a healthy dose, then reveal that a serving is four capsules, so a bottle lasts you a week. Work out the cost per day at your intended dose, not the sticker price. The cheap bottle is often the expensive one.
Watch for oxide hiding in the mix
Magnesium oxide is cheap and poorly absorbed, and it’s sometimes blended in quietly to bump up the elemental-magnesium number without adding much your body can use. If a blend leads with a well-absorbed form but lists oxide further down, be suspicious of how much of that headline number is doing anything.
Be wary of “proprietary blends”
If the label lists a blend but not the amount of each ingredient in it, you can’t tell what you’re getting. A brand confident in its formula usually tells you exactly how much of each form is in there. Vagueness is a choice.
The one that separates serious brands: third-party testing
Supplements aren’t approved for effectiveness before sale the way medicines are. The manufacturer is largely responsible for making sure the label is accurate. Most are fine, but quality varies more than you’d expect, and the only way to know an independent lab has checked what’s in the bottle, at the stated dose and without unwanted contaminants, is a third-party testing mark.
What the AUST L number on an Australian label means
If you’re buying in Australia, most magnesium supplements are listed medicines carrying an AUST L number on the label. It’s worth knowing exactly what that number does and doesn’t tell you, because it’s easy to read it as more than it is.
An AUST L listing means the product uses ingredients the TGA has already assessed as low risk, and that the sponsor has certified it’s manufactured to the required standard. It does not mean the TGA has evaluated whether the product works, and it does not mean anyone has independently tested that bottle’s contents.
The stronger mark is AUST R, for registered medicines, which are assessed for efficacy. You will rarely see it on a magnesium supplement.
So an AUST L number is a floor, not a recommendation. It tells you the product is legal to sell here and made under the right conditions. It tells you nothing about whether the form suits your goal or whether the dose is worth the money, which is what the rest of this page is for.
| Mark | What it checks | Notes |
|---|---|---|
| USP Verified | Identity, potency, purity, and that the tablet breaks down in your body | One of the strongest marks going |
| NSF (and Certified for Sport) | Label accuracy and contaminants; “for Sport” also screens banned substances | Matters if you’re a tested athlete |
| Informed Sport / Choice | Batch-by-batch testing | Aimed at athletes, a genuine quality signal for anyone |
| ConsumerLab | Buys products off the shelf and publishes its own test results | Independent, subscription-based |
Not every good product carries one. Testing costs money, and some solid smaller brands skip it. But when you’re choosing between two otherwise-similar bottles, a testing mark is a real tie-breaker.
One practical note for Australian buyers: USP and NSF marks are much less common on products sold here than on US ones, because the schemes are American and most Australian brands don’t pay into them. Their absence on an Australian shelf tells you less than it would in a US pharmacy. Informed Sport is the mark you’re most likely to actually encounter locally.
The caveat. Third-party testing tells you the bottle contains what it says, cleanly. It does not prove the supplement will do anything for you. That’s down to the form, the dose, and whether you were short on magnesium in the first place. A certified bottle of a poorly-chosen form is still the wrong bottle. Get the form right first, then use testing to choose the best version of that form.
The 30-second checklist
- The form matches your goal (sleep → glycinate, and so on).
- You’ve read the elemental magnesium per serving, not the front-of-pack number.
- You know how many capsules make a serving, and the real cost per day.
- There’s no undisclosed blend, and oxide isn’t padding the numbers.
- Ideally, a third-party mark (USP, NSF, Informed Sport, or ConsumerLab).
Get those five right and you’ve cleared the bar most of the shelf fails.
Sources: NIH Office of Dietary Supplements: Magnesium Fact Sheet (https://ods.od.nih.gov/factsheets/Magnesium-HealthProfessional/); U.S. Pharmacopeia (USP) Dietary Supplement Verification Program (https://www.usp.org/verification-services/dietary-supplements-verification-program); NSF Dietary Supplements Certification (https://www.nsf.org/consumer-resources/health-beauty/supplements-vitamins/dietary-supplements)
This article is general information, not medical advice. Regulation of supplements varies by country; check the rules where you live, and speak to a pharmacist or doctor before starting one, especially if you take other medication.