Calculator inputs
Example data
| Scenario | Category | Temp | RH | Wind | Leaf | Volume | Estimated effective rainfast | Recommended minimum dry time |
|---|---|---|---|---|---|---|---|---|
| Warm, breezy afternoon | Contact insecticide | 28°C | 45% | 10 km/h | Dry | 2.0 L/100m² | ~45–60 min | ~60–80 min |
| Cool, humid morning | Systemic fungicide | 16°C | 85% | 3 km/h | Damp | 2.5 L/100m² | ~4–6 hr | ~5–7 hr |
| Dense canopy in shade | Systemic herbicide | 22°C | 70% | 0 km/h | Dry | 3.0 L/100m² | ~5–7 hr | ~6–9 hr |
Formula used
This calculator starts with a base rainfast time from your product type, then applies multipliers for formulation, weather, leaf conditions, and application setup.
Multipliers are clamped to keep outputs within practical bounds. Always follow your label’s rainfast statement when provided.
How to use this calculator
- Select the product category and formulation closest to your label.
- Keep “Use suggested base time” on, unless you know the label value.
- Enter current temperature, humidity, wind, and leaf wetness.
- Add spray volume and any allowed adjuvant you plan to use.
- Optionally enter expected rain or irrigation timing for a risk check.
- Press Calculate, then plan spraying around the recommended dry time.
Why rainfast timing matters for garden results
Rainfast window and product performance
Rainfast time is the minimum drying and binding period needed before rainfall or irrigation can reduce coverage. If water hits too early, droplets may dilute, run off, or wash active ingredients from leaves. The calculator estimates an effective window and adds a safety buffer so you can plan sprays with fewer repeat applications.
Weather variables that change drying speed
Temperature, relative humidity, and wind directly influence evaporation. Cool air and high humidity slow drying, extending the rainfast window. Light airflow often helps, but strong wind increases drift and can create uneven deposition. For reliable results, prioritize calm periods with stable temperature and moderate humidity.
Leaf wetness, canopy, and surface effects
Dew, damp foliage, dense canopies, and shaded beds hold moisture longer. Waxy or hairy leaves can also keep droplets beaded, delaying film formation. When leaves are not fully dry, even a label-listed rainfast time may be optimistic. This is why the tool applies leaf and canopy multipliers.
Application setup and formulation considerations
Spray volume affects how much liquid must evaporate. Higher application rates usually require additional time. Formulations behave differently: granular products are less sensitive to rainfall, while oil dispersions can set quickly but must be used carefully. Stickers and compatible adjuvants can improve retention when allowed.
Planning around rain and irrigation schedules
Enter expected rain and irrigation timing to assess wash-off risk. A low-risk result means the forecast timing is later than the recommended minimum dry time. Borderline and high-risk outcomes suggest delaying spraying, reducing volume, choosing a better window, or extending the drying period to protect coverage and cost.
FAQs
1) Is this result the same as my product label rainfast time?
No. If the label provides a rainfast statement, treat it as primary. This tool estimates a practical window using conditions and a buffer, helping you plan when the label is silent or unclear.
2) What should I do if rain is expected sooner than the recommendation?
Delay spraying if possible. If you must spray, reduce spray volume, target the most urgent areas, and extend drying time with a longer gap before irrigation. Reapply only as the label permits.
3) Does morning dew change the rainfast window?
Yes. Dew or damp leaves slow drying and can dilute droplets, increasing wash-off risk. Use the “Damp” or “Wet” setting and consider waiting until foliage is fully dry for better coverage.
4) How does a sticker or adjuvant affect results?
When label-approved, stickers and certain adjuvants can improve retention and reduce wash-off. They are not universal solutions and may increase phytotoxicity risk on sensitive plants, especially in heat.
5) Why does high spray volume increase the estimated time?
More liquid on the leaf means more water must evaporate before a stable film forms. Heavy runoff also reduces uniform coverage. Use a calibrated sprayer and apply only to the point of good coverage.
6) Can I use this for overhead irrigation planning?
Yes. Enter your planned irrigation timing to see if watering is likely to occur after the safe window. If overhead watering is required earlier, consider drip irrigation or postponing treatment.