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Target pre-meal: 80-130 mg/dL

Timing Assessment

Select your injection timing and meal type above to see the impact.

How This Works

This calculator models the mismatch between rapid-acting insulin's onset (15-20 minutes) and a meal's glucose absorption rate (which varies by GI). When insulin is injected too late, glucose from digestion enters the bloodstream before insulin is available to process it - causing a temporary spike.

Peak BG ~ Pre-Meal BG + GI Spike - Insulin Coverage

GI spike contributions: Low GI: ~40 mg/dL | Medium GI: ~70 mg/dL | High GI: ~110 mg/dL

Timing offset: Each 15-minute delay in injection (relative to optimal) adds roughly 15-25 mg/dL to the estimated peak. Early injection (30 min before) with a low-GI meal can actually risk early hypoglycemia before the food absorbs.

These are illustrative estimates. Your actual response depends on your dose size, insulin type, absorption site, and individual variability.

Why Timing Matters

Rapid-acting insulin analogs (NovoLog, Humalog, Apidra) take 10-20 minutes to begin lowering blood glucose, with peak action at 60-90 minutes. Food digestion - especially high-GI foods - can raise blood glucose within 15-30 minutes of eating.

The pre-bolus: Injecting 15-20 minutes before eating (for rapid-acting insulin) allows insulin action to synchronise with glucose absorption. This "pre-bolus" timing is one of the most effective and underused tools for controlling post-meal spikes.

GI matters: For low-GI meals, glucose absorption is slow - you have more flexibility with timing, and an earlier injection may risk a low before the food is absorbed. High-GI meals require a more aggressive pre-bolus to match the rapid glucose rise.

Starting BG matters too: If your pre-meal BG is already high (>150 mg/dL), injecting earlier is even more important to avoid compounding the post-meal spike on top of an existing high.

Practical Tips by Meal Type

High-GI meals (white rice, bread, juice, sweets):

  • Inject 20-30 minutes before eating
  • Consider a slightly larger dose to match rapid absorption
  • Check BG at 1 hour post-meal

Medium-GI meals (pasta, oats, most fruits):

  • Inject 15 minutes before eating
  • Standard timing works well for most people

Low-GI meals (salads, legumes, vegetables, whole grains):

  • Inject at meal start or up to 10 minutes before
  • If pre-meal BG is below target, inject after eating to reduce hypoglycemia risk
  • Monitor for delayed rises (2-4 hours post-meal)
! Important Medical Disclaimer
This calculator provides educational estimates based on general population models and must not be used as the sole basis for insulin dosing or timing decisions. Injecting insulin too early (especially with low-GI meals or below-target pre-meal BG) carries a risk of hypoglycemia. Always use your actual blood glucose readings, CGM data, and the personalised guidance of your diabetes care team when making mealtime insulin decisions. Individual response to insulin timing varies considerably.