Half-life and elapsed time in the same units—fraction remaining ≈ (1/2)^(t / t½).
Remaining fraction = 0.5^(elapsed ÷ halfLife), shown as percent and raw fraction. Half‑life and elapsed time use the same time unit; half‑life must be > 0, elapsed ≥ 0 (at t = 0 you get 100%).
This calculator uses plain decimal numbers. If a result looks wrong, check whether you used a comma instead of a dot (or vice versa) where the field expects one.