Transferrin saturation (TSAT) calculator: iron ÷ TIBC

Transferrin is the protein that carries iron in your blood. Transferrin saturation, or TSAT, is the share of its iron-binding sites that are filled. It’s usually printed on an iron panel next to serum iron, TIBC, and ferritin, but some labs leave it off, and older reports often skip it. If you have serum iron and TIBC from the same draw, you can work it out. Enter both below, in either unit.

The formula

The American College of Gastroenterology hemochromatosis guideline defines it as serum iron divided by total iron-binding capacity:

TSAT (%) = serum iron ÷ TIBC × 100

Both values must be in the same unit. US labs report µg/dL (sometimes written mcg/dL); much of the world uses µmol/L. Iron’s molar mass is 55.845 g/mol, so:

µg/dL = µmol/L × 5.585

The calculator converts each field on its own, so a report that mixes units still works.

The first two rows are the pattern of iron deficiency: low iron and a high binding capacity, since the body makes more transferrin when it is short of iron. The fourth is the pattern that prompts a look for iron overload.

Reading the result

Two cutoffs matter most.

Below 20%. The British Society of Gastroenterology guideline treats a TSAT below 20% as a sign of iron restriction: too little iron is reaching the bone marrow to make red cells. That can mean empty stores, or it can mean inflammation is locking iron away even though stores are full. Ferritin is what tells the two apart, and the BSG calls it the single most useful test for iron deficiency. The ASH 2026 guidelines use TSAT below 20%, or ferritin below 100 ng/mL, to diagnose iron deficiency in adults with inflammation, where ferritin alone can read falsely normal.

Above 45%. The ACG guideline uses TSAT as the first screening test for hereditary hemochromatosis and reports that a value above 45% picks up 97.9% to 100% of people with the most common form (two copies of the C282Y gene variant). A ferritin in the normal range together with a TSAT below 45% makes iron overload unlikely. The EASL guideline uses above 45% for women and above 50% for men and postmenopausal women, each paired with a high ferritin.

Between 20% and 45% is where most results land. Your lab’s printed range may differ a little from these cutoffs, and the calculator’s reading is only a guide.

Why TSAT moves around

Ferritin is steady from week to week. Serum iron is not, and TSAT moves with it.

• Day to day. In a large screening study, Adams and colleagues tested people twice, months apart. A third of those with hemochromatosis had a TSAT below the usual cutoff at one of the two visits, enough within-person variation to miss them on a single draw.
• Time of day. Dale and colleagues drew iron studies at 8 AM, noon, and 4 PM. Morning iron was higher than afternoon iron in only half of their volunteers, and the swing between days was as large as the swing within a day. They concluded that limiting draws to the morning doesn’t make results more reliable.
• Fasting. The ACG guideline says fasting is not required for an accurate TSAT, and Adams found no advantage to fasting samples. If your lab asks for a fasting draw, follow its instructions anyway.
• Iron supplements. A dose of oral iron raises serum iron within hours. That rise is the basis of the oral iron absorption test, and one study counted a rise of more than 15 µmol/L (about 84 µg/dL) as normal absorption. A TSAT drawn a few hours after a supplement can read high for that reason alone, so ask whether to skip the dose before the draw.

The practical upshot is that one TSAT is a snapshot. A high value is worth repeating, and a TSAT reads best next to ferritin from the same draw.

Using transferrin instead of TIBC

Some labs measure transferrin directly, in mg/dL or g/L, and print no TIBC. Each transferrin molecule carries two iron atoms, so TIBC can be estimated from it. Gambino and colleagues compared the two in 570 samples and found the ratio close to the theoretical value:

TIBC (µmol/L) ≈ 25 × transferrin (g/L)

In US units that is about 1.4 µg/dL of TIBC per mg/dL of transferrin, so a transferrin of 250 mg/dL suggests a TIBC near 349 µg/dL. Enter transferrin in the optional field and the calculator shows a second TSAT from that estimate. It’s approximate: the errors of two different tests add together. If your report has a measured TIBC, use that.

When the calculator differs from your report

• Rounding. Labs calculate from unrounded iron and TIBC, so a difference of 1% is rounding.
• UIBC instead of TIBC. Some labs measure the unsaturated binding capacity and add serum iron to get TIBC. The result is the same; just use the TIBC line.
• Units. A TIBC of 60 is µmol/L, not µg/dL. The lab unit converter converts iron either way.

Tracking TSAT over time

TSAT is noisy, so the line is less useful than ferritin’s, but it still earns a place on the chart. A run of low values that rises with treatment, or a series that sits above 45% year after year, says more than any one draw. How to track your ferritin over time covers the iron panel as a whole, including the 2026 ferritin cutoffs and how inflammation hides a low result.

TSAT in HealthViewer

HealthViewer is a lab results tracker that runs on your own computer. It imports iron panels from lab report PDFs, MyChart downloads, and direct connections to your health system. A TSAT your lab prints is filed as Transferrin Saturation, and in Trends the Iron tag pulls together iron, ferritin, TIBC, and transferrin saturation.

If your lab doesn’t print TSAT, write a derived test: with A as Iron and B as TIBC, both in µg/dL, and a date window of 0 days, the expression is A / B * 100. It’s charted as its own line, apart from any TSAT your lab reports.

Everything stays in an encrypted file on your computer, with no account and no cloud service. Getting Started walks through the first import.

This calculator is for understanding your lab report, not for diagnosis or treatment. Talk to your clinician about an abnormal TSAT, and before starting or stopping iron.