Iron Saturation Calculator (Transferrin Saturation, TSAT %)

Calculate iron saturation (transferrin saturation, TSAT) from serum iron and TIBC, or find serum iron or TIBC from the other two, in µg/dL, µmol/L or mg/L.

Note: This calculator is for educational use only and is not medical advice or a diagnosis. Iron tests (including serum iron, TIBC, and transferrin/iron saturation) may help evaluate iron status when interpreted with other labs (such as ferritin and a CBC), your symptoms, and your clinician’s assessment; reference ranges vary by lab. Do not start/stop iron supplements based on this tool alone. Seek urgent care for severe symptoms (e.g., chest pain, fainting, black/tarry stools).

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Iron Saturation Formula

The following formula is used to calculate Iron Saturation.

IS = (SI / TIBC) \times 100

Variables:

IS is the Iron Saturation (%)
SI is the Serum Iron (commonly reported in ug/dL or umol/L)
TIBC is the Total Iron Binding Capacity (commonly reported in ug/dL or umol/L)

To calculate Iron Saturation, divide serum iron by total iron-binding capacity, then multiply by 100. Convert SI and TIBC to the same concentration unit first. This calculator uses reported TIBC, not transferrin protein concentration. Do not enter a transferrin result as TIBC; their units and laboratory calculations differ. See Mayo Clinic Laboratories’ iron and TIBC guidance.

What is Iron Saturation?

Iron saturation, clinically called transferrin saturation (TSAT), is a blood test that measures the percentage of iron-binding sites on transferrin that are occupied by iron. Transferrin is a glycoprotein synthesized primarily in the liver, and each molecule carries up to two ferric (Fe3+) ions. At any given time, only about one-third of circulating transferrin is loaded with iron, leaving a reserve binding capacity of roughly 67% (the unsaturated iron-binding capacity, or UIBC). The TIBC represents the sum of the iron already bound plus this reserve: TIBC = Serum Iron + UIBC.

TSAT is considered a more sensitive marker for real-time iron availability than serum iron or TIBC measured individually because it captures the ratio between supply and transport capacity. While ferritin reflects stored iron, TSAT reflects circulating iron available for erythropoiesis and enzymatic function at that moment.

Reference Ranges by Sex and Age

Reference ranges depend on age, sex, race, and the specific laboratory assay used. General adult ranges reported across major reference laboratories are as follows.

Use the reference interval supplied by the laboratory that performed the test. Intervals and clinical decision thresholds vary by method and clinical context; a calculator cannot establish a diagnosis from TSAT alone. Optional lab limits in this calculator compare only with the interval you enter.

Clinical Interpretation

A low TSAT may indicate reduced iron availability, but does not diagnose iron deficiency or iron-deficiency anemia by itself. Interpret it with ferritin, a CBC and clinical assessment.

An elevated TSAT may occur with iron overload or other conditions and may warrant further assessment with ferritin and other tests. A result above 50% does not mean transferrin is nearly fully saturated or establish the presence of non-transferrin-bound iron. ARUP’s hemochromatosis guidance describes further evaluation rather than diagnosis from this ratio alone.

A TSAT within a laboratory reference interval does not by itself exclude iron deficiency, iron overload or another disorder.

Iron Panel Patterns by Condition

Iron saturation is most useful when interpreted alongside the full iron panel. The pattern of results across serum iron, TIBC, TSAT, and ferritin helps differentiate conditions that may appear similar based on a single marker alone.

In iron deficiency anemia, serum iron is low, TIBC is elevated (the body upregulates transferrin production to capture more iron), TSAT is low (typically under 20%), and ferritin is low. In anemia of chronic disease (also called anemia of inflammation), serum iron is low, but TIBC is also low or normal because inflammation suppresses transferrin synthesis via hepcidin upregulation. TSAT may be low or borderline normal, and ferritin is often normal or elevated because it acts as an acute-phase reactant. In iron overload or hereditary hemochromatosis, serum iron is high, TIBC is low to normal, TSAT is elevated (often above 45 to 60%), and ferritin is elevated. In sideroblastic anemia, serum iron is normal to high, TIBC is normal to low, TSAT is elevated, and ferritin is elevated due to ineffective erythropoiesis trapping iron in mitochondrial ring sideroblasts. In acute liver disease, serum iron can spike dramatically as damaged hepatocytes release stored ferritin and iron, TIBC may rise, and TSAT can temporarily exceed normal ranges.

Hemochromatosis Screening Thresholds

Hereditary hemochromatosis (HH) is the most common inherited disorder among people of northern European ancestry. About 80% of cases are caused by homozygosity for the C282Y variant in the HFE gene, with prevalence ranging from approximately 1 in 83 in Ireland to less than 1 in 2,500 in southern Europe. The AASLD recommends a TSAT cutoff of 45% as an initial screening threshold, which identifies 97.9 to 100% of C282Y homozygotes. More specific thresholds are TSAT above 60% in males and above 50% in females, which identify iron metabolism abnormalities with roughly 95% accuracy.

Provisional iron overload criteria for confirmed C282Y homozygotes combine TSAT with ferritin: TSAT above 45% with ferritin above 200 ug/L in premenopausal females, or TSAT above 50% with ferritin above 300 ug/L in males and postmenopausal women. Despite high biochemical penetrance (about 80% of male homozygotes develop elevated ferritin), clinical penetrance, meaning actual organ damage, is much lower and variable. This is why TSAT serves as a gatekeeper test: elevated TSAT prompts HFE genotyping and ferritin monitoring, not immediate treatment.

Factors That Affect TSAT Results

Serum iron (and therefore TSAT) is one of the more volatile routine lab values. Multiple factors introduce variability that clinicians must account for when interpreting a single result.

Diurnal variation is significant. Serum iron follows a circadian rhythm with levels highest between approximately 8:00 AM and 3:00 PM, peaking around 11:00 AM in men and noon in women. Evening and overnight values can be substantially lower. This rhythm persists even in constant darkness, indicating it is driven by endogenous circadian clock genes rather than light exposure alone. Fluctuations of up to 17% from peak to trough have been documented.

Fasting status also plays a role. Blood collection after 5 to 9 hours of fasting provides a representative estimate, but overnight fasting of 12 or more hours can paradoxically elevate serum iron beyond usual levels. There is no consensus on whether fasting samples are superior to random samples for TSAT measurement, and practice varies between laboratories and clinical guidelines.

Inflammation is a major confounder. Hepcidin, a regulator of iron metabolism, can reduce iron release from macrophages and enterocytes, lowering serum iron and TSAT even when total body iron stores are adequate or elevated. Inflammation can raise ferritin while reducing circulating iron availability. Clinicians therefore interpret TSAT and ferritin together and may use additional tests; neither result provides a complete assessment alone.

Other factors that can shift TSAT include recent oral iron intake (can transiently spike serum iron), pregnancy (expanded plasma volume dilutes transferrin saturation), oral contraceptive use (increases transferrin production, raising TIBC and lowering TSAT), liver disease (affects transferrin synthesis), and nephrotic syndrome (urinary protein loss includes transferrin).

TSAT in Chronic Kidney Disease and Heart Failure

TSAT and ferritin are used together in CKD, but treatment thresholds depend on dialysis modality and clinical circumstances. In people with anemia and CKD G5 receiving hemodialysis, KDIGO 2026 recommendation 2.1 suggests initiating iron when ferritin is ≤500 ug/L and TSAT is ≤30%; criteria differ for people not receiving hemodialysis. These are clinician-guided treatment recommendations, not instructions to start supplements from this calculator. Heart failure has separate assessment and treatment criteria that a clinician applies with ferritin, TSAT, symptoms and heart function.