Ferrous sulfate vs ferrous gluconate: how to read an iron supplement label

Ferrous sulfate vs ferrous gluconate comes down to one number: elemental iron. A 325 mg ferrous sulfate tablet carries about 65 mg of it; a 325 mg ferrous gluconate tablet carries roughly 36–38 mg. The big number on the front of the bottle is salt weight, not iron.

Elemental iron sits on the panel, not the front label

Iron doesn't come as bare metal in a pill. It's bound into a salt or a chelate, and the whole compound weighs considerably more than the iron inside it. "325 mg" describes that compound. The fraction that is actually iron ranges from about 12% to nearly 100% depending on the form.

In the US, the Supplement Facts panel has to state iron as elemental iron with a % Daily Value beside it. The Daily Value for iron is 18 mg, so a 65 mg tablet prints somewhere around 361% DV. The front of the bottle is marketing space; the panel is the specification.

The ingredient line names the form, usually in parentheses: "Iron (as ferrous fumarate)" or "Iron (as ferrous bisglycinate chelate)". Two numbers and one form name is the entire label.

Ferrous sulfate vs ferrous gluconate, side by side

Ferrous sulfate heptahydrate is roughly 20% iron by weight, which is where the familiar 325 mg to 65 mg conversion comes from. Ferrous gluconate is roughly 12%, so an identical-looking 325 mg tablet lands near 36–38 mg — brands round differently, and panels print both figures. Same tablet weight, nearly twice the iron in one of them.

Dried (exsiccated) ferrous sulfate muddies this further. It's about 33% iron, so a 200 mg dried tablet and a 325 mg heptahydrate tablet deliver the same 65 mg. Different front numbers, identical panels.

Gluconate's reputation for being easier on the stomach is hard to separate from its smaller dose. Someone who switches from sulfate to gluconate and feels better has usually also cut their elemental intake by close to half. Take two gluconate tablets to close the gap and much of that advantage travels with it.

How the common iron forms compare

Form	Elemental iron by weight	Typical label	Reputation
Ferrous sulfate (heptahydrate)	~20%	325 mg = 65 mg iron	The research reference standard, and the benchmark for GI complaints
Ferrous sulfate, dried (exsiccated)	~33%	200 mg = 65 mg iron	Same iron, smaller tablet
Ferrous fumarate	~33%	325 mg = 106 mg iron	Most iron per milligram of the common salts
Ferrous gluconate	~12%	325 mg = 36–38 mg iron	Sold as easier to tolerate; also a smaller dose
Ferrous bisglycinate (chelate)	~20% as commonly supplied	25–28 mg iron per capsule	The usual "gentle iron"; an amino acid chelate
Carbonyl iron	~98–100%	45 mg = 45 mg iron	Milled iron powder; relies on stomach acid to dissolve
Polysaccharide iron complex	Label states elemental iron directly	150 mg iron per capsule is common	High-dose capsules; iron carried in a carbohydrate shell
Heme iron polypeptide	Small fraction by weight	~11 mg iron per tablet	Derived from bovine hemoglobin; small doses by design

Two rows behave unlike the rest. Carbonyl iron is elemental iron in the literal sense, so its front number and its panel number match — which makes 45 mg look trivial beside a 325 mg bottle when it is most of that bottle's iron. And heme iron supplements use the heme transport route rather than the non-heme route every ferrous salt takes, a distinction covered in heme vs non-heme iron.

"Gentle iron" is a claim, not a form

No standards body defines gentle iron. Manufacturers attach the phrase to bisglycinate, polysaccharide complex, carbonyl iron, and plain low-dose gluconate — four different chemistries under one label. The panel settles what the front cannot: a "gentle" capsule holding 25 mg of elemental iron is a quarter of a 106 mg fumarate tablet, and the dose is the variable most consistently tied to how iron feels.

The underlying mechanism is real. Iron supplement constipation, nausea, and dark stools track with unabsorbed iron reaching the lower gut, so anything that reduces the amount sitting there tends to reduce symptoms. The shaky step is from mechanism to any one product. Head-to-head trials at matched elemental doses are limited, and they don't point in one direction with much force. Iron bisglycinate may be better tolerated milligram for milligram — a defensible reading, not a settled one.

Reading any iron label in ten seconds

1. Find the "Iron" row in Supplement Facts. That number is elemental iron, always.
2. Check the serving size. "Per 2 capsules" doubles the arithmetic.
3. Read the form in parentheses on the ingredient line.
4. Use % DV as a cross-check: 18 mg is 100%.
5. Ignore the milligrams on the front unless you want to know the salt weight.

The same discipline applies to whatever gets written down afterward. "One iron tablet" throws away the only thing that changed when the brand changed; 65 mg or 38 mg keeps the day's total comparable to food and to how many milligrams a day you actually need. Iron Tracker is an iron intake tracking app for iPhone, and logging a supplement by its elemental milligrams rather than its tablet weight is the only way a bottle and a bowl of lentils add up to one honest number.

Where those numbers sit against the RDA

NIH's Office of Dietary Supplements sets the RDA at 8 mg/day for men 19–50 and women 51 and older, 18 mg/day for women 19–50, and 27 mg/day in pregnancy. The upper intake level for adults is 45 mg/day from all sources, and it isn't meant to apply to people taking iron under medical supervision. A single 65 mg sulfate tablet is therefore not a labeling error — it's a therapeutic dose, and which form and amount fit a given situation is a question for a clinician, not a label.

FAQ

How much elemental iron is in 325 mg of ferrous gluconate?

Roughly 36–38 mg. Ferrous gluconate is about 12% iron by weight, and manufacturers round to different figures, so panels for a 324–325 mg tablet commonly print 36 mg or 38 mg. The same 325 mg as ferrous sulfate is 65 mg.

Is ferrous gluconate gentler than ferrous sulfate?

Frequently in practice, largely because the tablet holds about half the iron. At matched elemental doses the gap between the two salts narrows and the evidence gets murky. Compare panels rather than front labels before deciding one form suits you better.

Does iron bisglycinate cause less constipation?

That's the main claim behind "gentle iron", and the reasoning is plausible: a chelate that stays intact through the gut leaves less free iron behind. Trials against ferrous sulfate at equal elemental doses are limited and mixed. Dose is still the more reliable lever.

Is carbonyl iron the same thing as elemental iron?

Almost. Carbonyl iron is high-purity iron powder, so its stated weight and its elemental iron are effectively one number. It dissolves slowly and depends on stomach acid, which is why 45 mg of carbonyl iron isn't interchangeable with 45 mg from a soluble salt.

Do heme iron supplements absorb differently?

Yes. Heme iron polypeptide enters through the heme pathway, so it's less sensitive to the phytates, calcium, and tannins that blunt non-heme absorption. Tablets are small — around 11 mg of elemental iron — so the panel matters as much here as anywhere.