T1D glossary

Insulin on board (IOB)

Also called active insulin, bolus on board.

Insulin on board is the insulin from earlier doses that a pump or app estimates is still working. Bolus calculators take it into account. On the Tandem t:slim, IOB is subtracted from the correction part of a bolus when glucose is above target. (OpenAPS documentation)

Formula

Correction after IOB (units) = correction insulin − IOB, not below zero (how a calculator applies IOB differs by device)

(Tandem Diabetes Care)

Worked example

Arithmetic only: the correction works out to 2 units, and the pump shows 1.2 units of IOB from a bolus 90 minutes ago. 2 − 1.2 = 0.8 units of correction remain.

Related terms

• Insulin sensitivity factor
• Bolus
• Carbs on board
• Closed loop

Sources, quoted

• OpenAPS documentation: “IOB - Insulin On Board, or insulin active in your body.”
• Tandem Diabetes Care: “Note: If Insulin on Board is present, it will only be subtracted from the correction portion of the bolus.”
• Tandem Diabetes Care: “Note: If Insulin on Board is present, it will be used to reduce the food bolus amount.”

Every quote above is checked against its source page before each update of this site; last checked 2026-10-06. The worked example is our own arithmetic. This page explains a term; it is not advice for a person's treatment.