Most off-rate calls come down to a tip that was sized for last year's speed, or a pressure that was pushed to make up the difference. Sizing a tip properly takes one formula and about two minutes. Here's the whole method — or skip the math and use our Nozzle & Orifice Selector, which does this against what we actually have on the shelf.
1. Decide what you need: GPA, speed, spacing
Gallons per acre comes off the chemical label or your agronomist — 10 GPA is common for burndown, 15–20 for fungicides and contact products where coverage matters. Speed is the ground speed you'll actually run, not what the sprayer can do. Spacing is the distance between nozzle bodies on the boom, usually 20 or 30 inches on a broadcast boom.
2. The formula
GPM per nozzle = (GPA × MPH × spacing in inches) ÷ 5,940
Example: 15 GPA at 12 mph on 20-inch spacing = (15 × 12 × 20) ÷ 5,940 = 0.61 GPM per nozzle.
3. Match that GPM to a tip size and pressure
Tip capacity is named by its flow at 40 psi, in tenths of a gallon per minute: an 02 tip flows 0.20 GPM at 40 psi, an 04 flows 0.40, and so on. The ISO color code is the same across brands:
- Orange 01 — 0.10 GPM at 40 psi
- Green 015 — 0.15
- Yellow 02 — 0.20
- Lilac 025 — 0.25
- Blue 03 — 0.30
- Red 04 — 0.40
- Brown 05 — 0.50
- Grey 06 — 0.60
- White 08 — 0.80
- Light blue 10 — 1.00
- Light green 15 — 1.50
For our 0.61 GPM example, a grey 06 at about 40 psi is right on it. A brown 05 would need roughly 60 psi to get there; a white 08 would be running down around 23 psi, which is too low for most tips to make a proper pattern. Aim for a tip that hits your GPM in the middle of its pressure range, so you have room to speed up and slow down without leaving the range.
4. The square-root rule
Flow does not scale with pressure — it scales with the square root of pressure. GPM2 = GPM1 × √(psi2 ÷ psi1). Doubling pressure from 30 to 60 psi only adds about 41% more flow, and it makes the droplets much finer. That's why a rate controller can't chase a big speed change on pressure alone: if you go from 8 mph to 14 mph on the same tip, the controller needs about 3 times the pressure to hold the rate, and drift goes through the roof. Change the tip instead.
5. When to move up or down a size
- You're consistently at the top of the pressure range at your normal speed → go up one size.
- You're below about 20–30 psi and the pattern looks streaky → go down one size (check the tip's minimum pressure — air-induction tips need more than standard flat fans).
- Switching from a contact product to a systemic or a drift-sensitive product → you may keep the size but change the style of tip for a coarser droplet.
- Changing spacing or speed for a new machine → run the formula again. A tip that was right at 20-inch spacing is 50% undersized at 30-inch.
6. Check it with a catch test
Set the pressure, hold a container under one nozzle for a timed interval, and compare the volume to the tip's chart. Do a few nozzles across the boom. Any tip flowing more than about 10% over the chart is worn — replace the set, not just the one. Worn tips are the quiet reason the tank runs out early.
Every TeeJet spray tip on our site carries the full Catalog 51A application-rate table (PSI, GPM, GPA at 4–20 mph) right on the product page, so you can confirm the number before you order. Start with spray tips, or run the selector and it'll shortlist what fits. Questions on droplet size or tip style for a specific product label — call (509) 838-2209.
Spray Center Electronics is an authorized TeeJet dealer in Spokane, WA. Tip capacities are nominal ISO values; confirm against the manufacturer's chart for the exact tip.