Sample spot scenarios
| Method | Spot details | Spots | Overspray | Volume per spot | Total volume |
|---|---|---|---|---|---|
| Area-rate | Circle 30 cm, 200 L/ha | 20 | 10% | 0.016 L | 0.319 L |
| Area-rate | Rectangle 20×40 cm, 1.5 gal/acre | 60 | 15% | 0.004 gal | 0.217 gal |
| Time-flow | 1.2 L/min, 3 sec per spot | 80 | 5% | 0.063 L | 5.040 L |
How the volume is calculated
- Convert your application rate to L/m².
- Compute spot area: circle π·(d/2)², rectangle l·w.
- Volume per spot: V = Rate(L/m²) × Area(m²) × (1 + overspray%).
- Total volume: Vtotal = V × number_of_spots.
- Convert nozzle output to L/min if needed.
- Volume per spot: V = Flow(L/min) × (time_sec/60) × (1 + overspray%).
- Total volume: Vtotal = V × number_of_spots.
- Mixing (optional): percent or dose multiplies total volume.
Practical steps for accurate spot spraying
- Pick your method. Use area-rate for coverage targets, time-flow for trigger timing.
- Measure a few spots. Average the diameter or length and width for better accuracy.
- Enter your spot count. If unsure, estimate high and add 5–15% overspray.
- Add mixing details. Choose percent or dose to estimate concentrate and water.
- Plan refills. Enter your tank size to see refill count and last fill volume.
- Export your report. Download CSV for logs or PDF for printouts.
Why spot spraying needs volume control
Spot treatments often fail because the delivered volume varies more than the chemistry. A difference of only two seconds on a trigger can double dose on small weeds. When you calculate volume per spot, you can keep coverage consistent across a whole bed, even when spot sizes change. Consistent volume also improves recordkeeping and avoids plant scorch from hotspots.
Choosing an application rate that matches labels
Many garden products list a coverage rate such as 200 L/ha or 1.5 gal/acre. Converting that rate to a per‑spot volume ties the label to your target. For example, a 30 cm circle at 200 L/ha needs about 0.015 L before overspray, so 50 similar spots require roughly 0.75 L. Adding 10% overspray lifts that batch to about 0.83 L.
Estimating spot area with quick field measurements
Field measurements do not need to be perfect, but they should be repeatable. Measure five spots, record diameter or length and width, then average. A 10% increase in diameter raises circle area by about 21%, so using an average prevents large errors when weeds vary. For rectangles, a 5 cm error on each side can shift area by 20% or more.
Nozzle timing calibration for repeatable doses
If you prefer timing, measure nozzle output at your working pressure. Fill a jug for 30 seconds, multiply by two, and you have L/min. Then set a target seconds‑per‑spot from the calculator, and practice that rhythm. Small changes in pressure or tip wear can shift output noticeably. Recheck output after changing filters, batteries, or lance length.
Reducing refills and leftovers with tank planning
Tank planning turns a volume estimate into an efficient workflow. Enter your sprayer capacity to see refills and the last fill amount. This reduces leftover mix that must be stored or discarded. It also helps you batch concentrate accurately, so each refill stays on‑rate and on‑target. Include a small rinse allowance if you flush lines between products after each application cycle.
Common questions
1. How do I choose between area-rate and time-flow?
Use area-rate when your product label specifies coverage per area. Use time-flow when you want a repeatable trigger time. If possible, compare both methods once; similar totals confirm your calibration is realistic.
2. What overspray percentage is reasonable?
For calm conditions and careful targeting, 5–10% is often enough. Use 10–20% when spots overlap, weeds are dense, or you expect extra rinsing. Lower the value after a few logged jobs match your estimate.
3. Can I use this for different sprayer tips?
Yes. Each tip has its own output at a given pressure. Re-measure nozzle output whenever you swap tips, filters, or batteries. Store the tip color or size in Notes so the CSV and PDF remain traceable.
4. Does the mix breakdown replace label directions?
No. It only estimates how much water and concentrate make the total spray volume. Always follow the label for allowable rates, safety gear, and re-entry guidance. If label directions conflict, the label should win.
5. Why are my field results higher than calculated?
Common causes are longer trigger time, higher pressure, larger spots than measured, and extra line priming. Measure a fresh set of spots, recheck nozzle output, and add a small allowance for rinsing or wind-driven overlap.
6. How should I log results for repeatability?
Save a PDF for each session and keep the CSV in a spray log. Record product name, tip, pressure setting, weather, and target size. Next time, reuse those inputs to keep dose consistent.