Insulin Bolus Calculator
Medically reviewed by Dr. Chhaya Makhija, MD, DipABLM — October 2026
Calculate your total mealtime bolus: carbohydrate component (from ICR) plus blood glucose correction (from ISF and your target). Does not model insulin on board — always account for IOB before dosing.
⚡ Bolus Estimator
Calculate ICR →
Calculate ISF →
Total Bolus Estimate
(Carbs ÷ ICR)
((BG − Target) ÷ ISF)
(IOB) — subtract manually
your IOB estimate
The Full Bolus Formula
The carb component covers the meal. The correction component adjusts for current glucose level — positive if above target, negative if below. IOB (insulin on board) is subtracted to prevent stacking. This calculator cannot model IOB automatically; subtract it manually or use a pump/CGM system that tracks it.
IOB stacking is a leading cause of hypoglycemia. If you gave a correction bolus within the last 2–4 hours (depending on your insulin type), active insulin remains. Always account for it before giving another dose. Most insulin pumps and CGM systems calculate IOB automatically — use that if available.
What goes into a mealtime bolus
A bolus is the dose of rapid-acting insulin you take around a meal, and it's really two calculations rolled into one. The first part covers the carbohydrates you're about to eat; the second nudges your glucose back toward target if you're starting high. Adding them together gives the total bolus — which is exactly what this calculator does.
The carbohydrate part
This is your carb count divided by your insulin-to-carb ratio. If one unit covers 10 grams and you're eating 60 grams, that's six units just to handle the food. Accurate carb counting is the single biggest driver of a good post-meal number.
The correction part
If your pre-meal glucose is above target, a correction is added: the gap between your current and target reading, divided by your correction factor. If you're in range, this part is zero; if you're low, it can even reduce the meal dose.
When to inject a bolus relative to the meal
A bolus works best when insulin and food reach the bloodstream together. Inject too late and glucose spikes before the insulin acts. Inject too early and glucose falls before the meal lands. Each product carries its own timing instruction on the label.
| Insulin | Label timing | Practical note |
|---|---|---|
| Lispro (Humalog) | Within 15 minutes before a meal, or right after | The usual default for rapid analogues |
| Aspart (NovoLog) | 5–10 minutes before a meal | Peaks 1–3 h, lasts 3–5 h |
| Faster aspart (Fiasp) | At the start of the meal, or up to 20 minutes after | Suits people who cannot predict intake |
| Regular human insulin | About 30 minutes before a meal | Slower onset needs a longer lead |
Timings come from the manufacturers’ prescribing information. Check the label supplied with your own product.
Extended and dual-wave boluses
Pumps deliver a bolus over time instead of all at once. An extended or square-wave bolus spreads the dose across a set period. A dual-wave bolus gives part immediately and spreads the rest. Both exist because fat and protein slow digestion, so pizza or steak raises glucose hours after the carbohydrate clears. Injection users approximate this by splitting a dose, which carries more stacking risk and belongs in a conversation with your care team first.
Avoiding insulin stacking when you correct
The most common way a correction goes wrong is "stacking" — adding a new dose while an earlier one is still active. Rapid-acting insulin keeps working for roughly three to five hours, so if you correct a high an hour after your meal bolus, much of that meal insulin is still in your system. Stack the two and you can overshoot into a low. The safeguard is to account for your insulin on board before adding any correction, and to give a previous dose time to finish before judging whether you really need more.
Why your bolus changes from meal to meal
No two boluses are quite the same, because the inputs shift constantly. The carb content of the meal, your starting glucose, the time of day (many people are more insulin-resistant at breakfast), recent or planned exercise, and even fat and protein content all move the right dose up or down. That's the strength of calculating each bolus rather than using a fixed number — it adapts to the meal in front of you. The figure here is a careful estimate; confirm your ratios and correction factor with your care team, and let your post-meal readings refine them. Basal and supply questions are answered by the other insulin calculators on our home page.
Frequently Asked Questions: Insulin Bolus Calculator
How do I use the insulin bolus calculator?
Enter your meal carbs, insulin-to-carb ratio, current and target blood glucose, and sensitivity factor. It returns the carb bolus, the correction bolus, and the combined total — then subtract any insulin on board before dosing.
What's the difference between a bolus and a basal dose?
Bolus is the rapid-acting insulin for meals and corrections; basal is the long-acting background insulin. This tool covers the bolus — estimate background insulin with the Basal Insulin Calculator.
When should I take my bolus insulin?
Rapid-acting bolus insulin is usually given 0–15 minutes before eating so it peaks with the meal; ultra-rapid versions can be taken at the start of the meal, and Regular insulin needs about 30 minutes' lead time. Follow your prescribed timing.
Does this calculator subtract insulin on board?
No — it shows the carb and correction components but does not auto-subtract IOB. Estimate active insulin with the IOB Calculator and subtract it, or use a pump/CGM system that tracks IOB.
Source
- Walsh J, Roberts R. Pumping Insulin. 5th ed. 2012.
- American Diabetes Association. Standards of Care in Diabetes — 2026. Section 9. Link
Written by Ryan Mitchell · Last updated: September 2026