Model residue decline from half-life and wash-off events. Compare scenarios for beds, pots, and lawns. Get clear remaining dose and next spray date today.
| Scenario | Area (m2) | Days | Half-life (days) | Sun (h/day) | Water (mm) | Initial (mg/m2) | Remaining (%) | Reapply day |
|---|---|---|---|---|---|---|---|---|
| Vegetable bed, mild weather | 80 | 7 | 10 | 6 | 10 | 45 | ~40–55 | ~10–14 |
| Leafy crop, hot and sunny | 60 | 5 | 8 | 9 | 5 | 50 | ~35–50 | ~7–10 |
| Hardscape edges, heavy rain | 120 | 3 | 12 | 5 | 30 | 40 | ~20–40 | ~4–7 |
1) Base decay (first‑order):
k = ln(2) / half_life, and residue(t) = residue0 × e^(−k_eff × t)
2) Environmental adjustments:
3) Wash-off from water:
Each 10 mm reduces residue by a chosen percentage. Over elapsed days, water is distributed evenly in the model.
Residual control is the useful protection that remains after spraying, not the initial knockdown. It depends on how much active ingredient deposits on the target, then how quickly that deposit breaks down or washes away. Coverage quality matters too, because missed undersides and dense canopies reduce effective residue. This calculator separates those effects so you can compare sites and seasons using the same logic.
Half-life is the time for residue to drop to half under reference conditions. If you only have a rough estimate, test a low, typical, and high half-life to bracket outcomes. Short half-lives are common on exposed foliage, while protected surfaces often retain longer. When unsure, keep notes from repeat applications and refine inputs over time.
Breakdown often accelerates in warm weather. The Q10 factor estimates how much the decay rate changes for each 10 °C shift from your reference temperature. Sunlight can also speed loss through photolysis, so hours of sun and UV sensitivity help reflect summer beds versus shaded borders. Use conservative settings for young seedlings or heat-stressed plants.
Water exposure removes residue mechanically. The wash-off setting expresses the percent loss per 10 mm of rain or overhead watering and is distributed over your elapsed days. Drip systems may justify a lower wash-off, while storms, sprinklers, or frequent hand watering justify higher values. Surface factors represent typical retention differences between soil, mulch, foliage, and hardscape. Add a small safety margin when weather forecasts are uncertain.
A threshold is the minimum residue level you consider effective for your target pest. Setting a clear threshold converts the curve into a practical “days to reapply” estimate. Respect minimum label intervals and rotate modes of action to reduce resistance pressure. Always confirm with field scouting, because pest pressure, coverage, and microclimates can shift real performance. Combine the estimate with monitoring traps for added confidence.
It is the estimated deposit still present on the treated surface after decay and wash-off. It is not a safety limit, and it does not replace label guidance or local regulations.
Use “From spray mix” when you know tank dose, active percent, and area covered. Use “Direct initial residue” when you have measured or trusted starting residue values.
Choose a percent that matches your action level for the pest, crop, and risk tolerance. Many users start around 20–30% and adjust based on scouting and past outcomes.
Rain can physically remove residues, especially on smooth leaves or hardscape. If wash-off is high and rainfall is frequent, reapplication timing is driven more by water than by half-life.
Yes, approximate it by lowering total water exposure and/or wash-off sensitivity. If foliage stays dry, wash-off can be minimal; if water splashes leaves, increase wash-off accordingly.
No. Always follow product labels, restricted entry intervals, and harvest timing. Use this tool to compare scenarios and plan scouting, not to override legal or safety requirements.
Important Note: All the Calculators listed in this site are for educational purpose only and we do not guarentee the accuracy of results. Please do consult with other sources as well.