Long-Acting Insulin Calculator
Medically reviewed by Dr. Chhaya Makhija, MD, DipABLM — October 2026
Estimate your long-acting (basal) insulin dose from TDD for glargine, detemir, and degludec. Includes timing, frequency, and duration reference for each insulin type. Educational use only.
⏱️ Long-Acting Dose Estimator
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Long-Acting Dose Estimate
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Long-Acting Insulin Comparison
| Insulin | Brand | Onset | Duration | Peak | Dosing |
|---|---|---|---|---|---|
| Glargine U-100 | Lantus, Basaglar | 2–4 hr | ~24 hr | Peakless | Once daily |
| Glargine U-300 | Toujeo | 6 hr | ~36 hr | Peakless | Once daily |
| Detemir | Levemir (discontinued in the US end-2024) | 1–2 hr | 18–22 hr | Mild peak | Once or twice daily |
| Degludec | Tresiba | 1 hr | >42 hr | Peakless | Once daily (flexible timing) |
| NPH | Humulin N, Novolin N | 2–4 hr | 12–16 hr | 4–10 hr peak | Twice daily |
Glargine U-300 and degludec provide flatter, longer action profiles with less nocturnal hypoglycemia risk compared to glargine U-100. Detemir often requires twice-daily dosing at higher doses. NPH has a pronounced peak and greater hypoglycemia risk — now largely replaced by the newer analogues except in cost-constrained settings.
How to titrate (adjust) a long-acting insulin dose
Calculating the starting dose is the easy half — the dose that actually holds your fasting glucose in range is found by titration. The standard approach: change one thing at a time, measure fasting glucose every morning, and give each change time to reach steady state before judging it.
| Insulin | Typical adjustment | Steady state | Wait before judging |
|---|---|---|---|
| Glargine U-100 (Lantus, Basaglar) | 2 units every 3 days, or ~10–20% weekly | 2–4 days | 3 days minimum |
| Glargine U-300 (Toujeo) | Same step size, judged against fasting readings | ~5 days | 5 days — a change on day 2 has not finished arriving |
| Detemir (Levemir — discontinued in the US end-2024) | 2 units every 3 days; expect twice-daily at higher doses | 2–3 doses | 2–3 days |
| Degludec (Tresiba) | Small steps; its 42 h action smooths day-to-day | 3–4 days | 4 days |
Titration safety rules: raise the dose only if fasting glucose runs above target for several mornings in a row — not after one high reading. If a dose increase causes night-time lows (sweating, headaches, vivid dreams), step back one increment and hold. Never adjust during an illness, after a steroid start, or after exercise changes without accounting for them first. Confirm your titration schedule with your prescriber — these are the commonly published protocols, not a personal instruction.
The steady-state column matters more than people expect. Toujeo needs about five days to reach full effect, so reacting to a dose change on day two means judging an insulin that has not finished arriving — and stacking increases on top of each other is how nocturnal hypoglycemia happens days later, with no obvious cause on the night it strikes.
How long-acting insulin keeps glucose steady between meals
Long-acting insulin — glargine, detemir, and degludec — is the background layer of your regimen. Its job isn't to cover meals but to balance the glucose your liver releases steadily throughout the day and overnight. The defining feature of these insulins is that they're designed to be relatively "peakless": instead of surging and fading like rapid-acting insulin, they release slowly and evenly, so your fasting and between-meal numbers stay flat rather than drifting.
That smooth profile is what makes long-acting insulin the foundation a basal-bolus regimen is built on. Get the background dose right and your mealtime insulin has a stable baseline to work from; get it wrong and no amount of bolus tinkering will fully settle your glucose.
Once-daily or twice-daily: which long-acting insulin suits you
Not all long-acting insulins last equally long, and that shapes how often you take them. Degludec is ultra-long-acting and comfortably covers a full day or more from a single injection. Glargine usually works once daily for most people, though some need it split. Detemir is shorter-acting and is frequently given twice a day to avoid an afternoon gap.
Signs a single daily dose isn't lasting
If your glucose is well controlled for most of the day but climbs reliably before your next injection is due, your basal insulin may be wearing off early. Splitting the same total into two doses often smooths that out — but it's a change to make with your provider, not on your own.
Why taking it at the same time each day matters
Long-acting insulin works best on a consistent schedule. Because each dose is meant to hand over to the next without a gap, taking it hours late — or missing it entirely — leaves a window where background coverage fades and glucose rises. The exact time of day matters less than the consistency, so most people anchor the injection to a fixed daily habit, like brushing their teeth at night. The dose estimate here is a starting point; your fasting readings and your care team determine the final number. Mealtime and correction doses are worked out with our bolus and correction calculators.
Long-Acting Insulin Calculator: Limitations & Safety Notes
- Long-acting insulins are not interchangeable unit-for-unit. Glargine U-100, glargine U-300 and degludec differ in duration and peak profile, and switching usually requires a dose adjustment.
- Basal insulin is titrated against fasting glucose, conventionally no faster than every three days, because it takes that long for a change to show its full effect.
- A high fasting reading is not always insufficient basal. Dawn phenomenon and post-hypoglycemic rebound both look the same on a morning meter, and raising basal in either case risks nocturnal hypoglycemia.
- Basal insulin does not cover meals. Persistent post-meal highs are a bolus problem, not a basal one.
Frequently Asked Questions: Long-Acting Insulin Calculator
What is long-acting insulin used for?
Long-acting (basal) insulin provides steady background coverage for 18 to 42+ hours, controlling glucose between meals and overnight. It is paired with rapid-acting bolus insulin for meals in a basal-bolus regimen.
How is a long-acting insulin dose estimated?
A common starting basal dose is about 50% of total daily insulin, or roughly 0.1–0.2 units/kg/day when starting basal alone in type 2 diabetes. The dose is then titrated to your fasting glucose with your care team.
Which long-acting insulin lasts the longest?
Can long-acting insulin be taken twice a day?
Detemir and NPH are often split into two daily doses, especially at higher doses. Glargine and degludec are usually once daily. Levemir (detemir) was discontinued in the US at the end of 2024, so people still taking it are usually switched to glargine or degludec. Your prescriber sets the schedule based on your glucose pattern and the specific insulin.
Sources
- American Diabetes Association. Standards of Care in Diabetes — 2026. Section 9. Link
- Heise T et al. "Pharmacological properties of insulin degludec and insulin glargine." Drug Des Devel Ther. 2015.
Written by Ryan Mitchell · Last updated: October 2026