The Short Version
The number printed in big type on a magnesium glycinate bottle, "400mg," "500mg," "1,000mg," is almost always the weight of the entire magnesium glycinate compound, not the amount of actual magnesium it delivers. Magnesium glycinate is roughly 14 percent elemental magnesium by weight, so a "400mg" capsule typically provides only about 56 to 60mg of the magnesium your body can use. The dose most people are aiming for, 200 to 400mg of elemental magnesium per day, is measured in that smaller number, not the big one on the front.
This is the single most common way magnesium shoppers overpay or underdose. A product that looks like a high-dose bargain at "400mg per capsule" can quietly require three or four capsules to reach a real serving. Here is the simple chemistry, how to find the number that matters on the label, and how to compare products honestly. For our scored picks, see the magnesium glycinate scorecard.
Elemental Magnesium vs the Compound: What the Big Number Means
Magnesium does not exist on its own in a supplement. It is bound to a carrier molecule, and in magnesium glycinate (also written magnesium bisglycinate) that carrier is glycine, an amino acid. The finished compound is heavy: two glycine molecules wrapped around one magnesium atom. Magnesium itself is the small part of that total weight.
Run the numbers and magnesium accounts for about 14 percent of the bisglycinate molecule by weight. So:
- A "1,000mg magnesium glycinate" capsule provides roughly 141mg of elemental magnesium.
- A "400mg" capsule provides roughly 56 to 60mg of elemental magnesium.
- A "500mg" capsule provides roughly 70mg of elemental magnesium.
The "elemental" amount is the one your body counts. Every dosing recommendation, every clinical trial, and the official intake figures from the NIH Office of Dietary Supplements are expressed in elemental magnesium. The compound weight on the front of the bottle is just marketing-friendly because it is a bigger number.
Why This Matters for Your Dose
The everyday target for supplemental magnesium is about 200 to 400mg of elemental magnesium per day, and the sleep and blood-pressure research that makes glycinate popular used doses in that elemental range. It is also worth knowing that the Institute of Medicine set a tolerable upper limit of 350mg of elemental magnesium per day from supplements for adults, so chasing an enormous front-label number is not the goal anyway.
Now put that next to a "400mg" capsule that actually delivers 60mg elemental. Take one a day and you are at 60mg, well short of a clinical dose. To reach 200mg you need a bit more than three capsules. That is not necessarily a bad product, but it is a very different proposition than "400mg per capsule" implies, and it changes both how many capsules you swallow and what the bottle truly costs per effective dose.
A Real Example: the "400mg" That Is Really 60mg
One of the best-selling magnesium glycinates on Amazon, Double Wood Magnesium Glycinate, is labeled "400mg." Read its Supplement Facts panel and each capsule lists about 60mg of elemental magnesium, exactly what the 14 percent chemistry predicts. The brand does disclose the 60mg figure on the panel; in our view the front label simply leads with the 400mg compound weight, which is the number most shoppers read and remember.
The practical effect: the company's own directions suggest one to two capsules, and reaching a clinical 200mg dose takes roughly three. So a bottle that looks cheap per capsule lands around $0.28 per effective 200mg dose once you account for the elemental content, which is mid-pack, not a bargain. We walk through this and every other product on the magnesium glycinate scorecard, where we score cost on the elemental dose you actually absorb, not the label weight.
How to Read a Magnesium Label in 10 Seconds
You do not need to do chemistry at the shelf. The Supplement Facts panel is required to list the elemental amount. Here is the quick routine:
- Ignore the big front-of-bottle number first. "400mg," "500mg," and "1,000mg" usually describe the compound, not the magnesium.
- Find the "Magnesium" line in the Supplement Facts panel. The figure next to "Magnesium" that has a % Daily Value beside it is the elemental amount. The %DV is based on 420mg, so a product showing "100mg, 24% DV" is giving you 100mg of elemental magnesium.
- Check the serving size. That elemental number is per serving, and a serving might be two, three, or four capsules, so confirm how many pills it takes to get there.
- If only the compound weight is listed, do the math. Multiply the compound milligrams by about 0.14 to estimate elemental magnesium (400 x 0.14 is about 56mg).
If a label brags about a big milligram number but you cannot find an elemental "Magnesium" line with a %DV, treat that as a yellow flag, not a selling point.
Different Forms Are Labeled Differently, So Compare Carefully
The elemental percentage changes with the magnesium form, which makes cross-form comparison misleading if you only read the front label.
- Magnesium glycinate / bisglycinate: about 14 percent elemental. Gentle on the stomach and the form most people want for sleep and relaxation. See which magnesium form is best.
- Magnesium oxide: about 60 percent elemental, so the front number looks efficient, but it is poorly absorbed and mostly acts as a laxative. A high elemental number does not help if little of it gets in.
- Magnesium citrate: about 16 percent elemental, well absorbed, with a mild laxative effect.
- Magnesium L-threonate: only about 8 percent elemental, so a "2,000mg" threonate product delivers a deliberately small ~144mg of elemental magnesium. That is by design, since it is sold for brain-targeted reasons rather than total magnesium. See our magnesium L-threonate scorecard.
This is also why "more elemental magnesium per pill" is not automatically better: oxide wins that contest on paper and loses it in your gut. Absorption and tolerability matter as much as the raw number, a point we cover in our guide to supplement bioavailability.
How We Score It
Because the label number and the usable dose can differ by a factor of seven, we normalize every magnesium product to its elemental content and its cost per effective dose, meaning the price to reach a clinical 200 to 400mg of elemental magnesium, not the price per capsule. That is how a "400mg" bestseller and a plainly labeled "200mg elemental" capsule end up in a fair fight. The same trap appears in turmeric, fish oil, collagen and more, which we cover in the supplement label trap. The value winners on our scorecard and our best magnesium for sleep guide are the ones that hit a real elemental dose for the least money, not the ones with the biggest front-label number.
The Bottom Line
The "400mg" on a magnesium glycinate bottle is usually the weight of the whole glycinate compound, and only about 14 percent of it, roughly 56 to 60mg, is the elemental magnesium your body uses. Aim for 200 to 400mg of elemental magnesium per day, find that elemental number on the Supplement Facts panel (it is the one with the % Daily Value next to it), and compare products on the dose you actually absorb. Do that and the flashy front-label number stops fooling you, and the genuinely good value becomes easy to spot. For the scored picks, start with the magnesium glycinate scorecard.
These statements have not been evaluated by the FDA. This product is not intended to diagnose, treat, cure, or prevent any disease.