- Pressure scaling: Qactual = Qref × √(Pactual / Pref)
- Broadcast rate (US): GPA = (5940 × GPMper nozzle) / (MPH × spacingin)
- Required flow (US): GPMreq = (GPAtarget × MPH × spacingin) / 5940
- Estimated pressure: Preq = Pref × (GPMreq / GPMref)²
- Tank coverage: acres per tank = tank gallons / GPA (uses target if provided)
- Find your nozzle flow at a known reference pressure.
- Enter your operating pressure and boom nozzle spacing.
- Add your real travel speed, measured in the field.
- Optional: enter a target rate to back-calculate needed flow.
- Optional: add tank volume to estimate coverage per fill.
- Press Calculate and review per-nozzle, total, and rate checks.
| Nozzle rating | Pressures | Speed & spacing | Outputs |
|---|---|---|---|
| 0.40 gpm @ 40 psi | Actual: 50 psi | 4.5 mph, 20 in, 12 nozzles | 0.447 gpm/nozzle, 5.36 gpm total, ~29.5 GPA |
| 1.20 L/min @ 3.0 bar | Actual: 2.5 bar | 6.0 km/h, 50 cm, 8 nozzles | 1.095 L/min/nozzle, 8.76 L/min total, ~219 L/ha |
| 0.25 gpm @ 40 psi | Target: 15 GPA | 5.0 mph, 20 in, 10 nozzles | Req: 0.253 gpm/nozzle, est. 41 psi, 2.53 gpm total |
Why flow rate matters in garden spraying
Flow rate controls how much solution reaches leaves and soil per minute. When flow is too high, runoff increases and product is wasted. When it is too low, coverage becomes patchy and pests survive. This calculator links nozzle rating, operating pressure, speed, and spacing to quantify delivery.
Pressure, nozzle output, and drift control
Nozzle charts list a reference flow at a reference pressure. Actual flow changes with the square‑root relationship: doubling pressure does not double flow, it raises it by about 41%. Higher pressure can create finer droplets and drift, while lower pressure can reduce coverage. Use the pressure estimate to hit a target rate without exceeding your nozzle’s recommended range.
Speed and spacing set the application rate
Application rate depends on ground speed and nozzle spacing just as much as nozzle output. The broadcast equation uses a constant (5940) to convert minutes, miles, inches, and gallons into gallons per acre. Slower travel increases rate, wider spacing increases rate, and faster travel decreases rate. Consistent speed is essential; measure it over a known distance with a full tank.
Total boom flow and pump planning
Multiply per‑nozzle flow by the number of nozzles to estimate total boom demand. Add additional circuit flow if you run agitation, return lines, or hand wands. Compare the total to your pump’s rated output at operating pressure to ensure stable pressure regulation. If total demand is close to pump capacity, pressure will fluctuate when valves open or close.
Tank coverage, mixing accuracy, and recordkeeping
With a known tank volume and rate, you can estimate acres or hectares per fill and scale chemical mixing accurately. If you enter a target rate, coverage is based on that target; otherwise it uses the calculated rate. Exporting results to CSV or PDF supports spray logs, compliance notes, and repeatable calibrations across seasons and nozzle replacements. Store nozzle model, lot number, and weather conditions so future adjustments are based on evidence, not guesswork alone.
1) How do I find nozzle flow at reference pressure?
Look up your nozzle size and spray angle on the manufacturer chart. Choose the listed flow at a stated reference pressure, then enter both values here to scale flow to your operating pressure.
2) Why does the calculator use a square‑root pressure relation?
Most spray nozzles behave like an orifice. Flow changes with the square root of pressure ratio, so modest pressure changes fine‑tune rate without extreme shifts in output.
3) What spacing should I enter for a hand wand?
Use the effective swath width you maintain while walking. Measure the average band width on the ground, then enter that spacing so the rate reflects your real coverage.
4) How can I verify the computed rate in the field?
Perform a timed catch test. Collect output from one nozzle for one minute, compare to the predicted per‑nozzle flow, and adjust pressure or speed until they match.
5) Does total flow include agitation or bypass return?
No. The total shown is boom demand only. Add extra circuit flows when checking pump capacity, especially for hydraulic agitation, recirculation, or a spray gun.
6) Which target rate should I follow, GPA or L/ha?
Use whatever your label or plan specifies. The calculator converts between units, so pick the unit you work with and keep it consistent across mixing and logging.