Herbicide goes in by the acre, not by the gallon of water. Divide your tank size by your calibrated gallons per acre to get acres per tank, then multiply by the label rate. A 300-gallon tank at 15 GPA covers 20 acres, so a 2 pint per acre rate needs 40 pints (5 gallons) of product.
That is the whole herbicide tank mix calculator, and every extension service teaches it the same way. The division only works if two numbers are true: the GPA your sprayer actually puts down, and the acres you actually have. Get those right and the rest is arithmetic, including the cases a basic calculator skips.
Every rate below is a worked example of the arithmetic, not a recommendation. Use the rate on your product label.
Three steps, in this order:
Ohio State's worked example: a 300-gallon tank calibrated at 15 GPA covers 300 ÷ 15 = 20 acres. At a label rate of 2 pints per acre, that is 20 × 2 = 40 pints (5 gallons) of product, plus 295 gallons of water, not 300. The product takes up room in the tank. (Ohio State FABE-530)
Here is the idea that matters most: if your GPA changes, the product per tank changes. The product per acre does not. Drop from 15 to 10 GPA and the same 300-gallon tank covers 30 acres, so it now needs 60 pints. The herbicide follows the acres, and the water just carries it there.
| Liquid | Equals |
|---|---|
| 1 gallon | 4 quarts = 8 pints = 128 fl oz |
| 1 quart | 2 pints = 32 fl oz |
| 1 pint | 16 fl oz |
Keep fluid ounces (volume) and ounces (weight) apart. Dry products such as WG and DF formulations are labeled by weight, so measure them on a scale, not in a jug.
Every number downstream of GPA inherits its error. If the sprayer is really putting down 17 GPA when you assumed 15, a 300-gallon tank covers about 17.6 acres instead of 20, and every acre gets roughly 13 percent more product than the label rate. Worn nozzles and a ground speed that drifted from the one you calibrated at are the usual causes. Most "tank mix errors" are calibration errors.
The quickest field method is the 1/128-acre method, sometimes called "ounces equal gallons." You drive a course equal to 1/128 of an acre per nozzle, then catch one nozzle's output for the same number of seconds. Because a gallon is 128 fluid ounces, the ounces you catch equal your gallons per acre. (UGA Extension C683), (LSU AgCenter)
Course length depends on nozzle spacing:
| Nozzle spacing | Course length for 1/128 acre |
|---|---|
| 20 in | 204.2 ft |
| 24 in | 170.2 ft |
| 30 in | 136.1 ft |
| 38 in | 107.5 ft |
(The course is just 1/128 of an acre, 340.3 square feet, divided by the nozzle spacing in feet.)
Time the drive in the field you will spray, at spraying gear and throttle, and average two runs. Then, parked at spraying pressure, catch several nozzles for that many seconds. A tip putting out noticeably more than its neighbors is worn.
What moves GPA, and by how much: doubling ground speed halves GPA. Pressure is weak by comparison, because flow follows the square root of pressure, so you have to quadruple pressure to double the rate. (Cal-IPC 2023, UC Cooperative Extension) Make big changes with nozzles or speed and use pressure for fine-tuning only, since it also changes droplet size and drift. If you change speed, redo the timed drive. If you change only pressure or tips, just re-catch.
Pulse-width rigs hold rate across speed changes within a range; our PWM nozzle control guide covers the limits.
Some labels and many extension recommendations give the rate as pounds of active ingredient (a.i.) or acid equivalent (a.e.) per acre, not as product. Convert to product first, then run the tank math.
(Ohio State FABE-530), (UMN Extension)
The formulation code on the jug tells you the strength. A "3ME" is 3 lb a.i. per gallon, microencapsulated. A "90DF" is 90 percent a.i., dry flowable. (SDSU Extension, 2026)
Glyphosate products are sold at 3, 3.7, 4, 4.17, 4.5 and 5 lb acid equivalent per gallon. (UC ANR Weed Science) For a 0.75 lb a.e. per acre rate:
| Product strength | Product per acre |
|---|---|
| 3 lb a.e./gal | 32 fl oz |
| 4.5 lb a.e./gal | about 21 fl oz |
Same dose, an 11-ounce difference per acre. Carry "a quart an acre" from one jug to the next and you either short the weeds or overspend by half.
Refilling a tank that is not empty. Base the math on the gallons you are adding, not the tank size. If 75 gallons of mix are still in the 300-gallon tank, you are adding 225 gallons: 225 ÷ 15 GPA = 15 acres, × 2 pints = 30 pints (3.75 gallons) of product, plus 221.25 gallons of water. (Ohio State FABE-530)
Finishing a field. Fields are not round numbers of tanks, and this is where leftover mix comes from. Scale water and product together to the acres left.
Worked example (arithmetic only): a 47.3-acre field, 300-gallon tank, 15 GPA, 2 pints per acre.
| Load | Spray volume | Acres | Product |
|---|---|---|---|
| Tank 1 | 300 gal | 20 | 40 pt (5 gal) |
| Tank 2 | 300 gal | 20 | 40 pt (5 gal) |
| Last load | 109.5 gal | 7.3 | 14.6 pt (about 1.8 gal) |
| Total | 709.5 gal | 47.3 | 94.6 pt (about 11.8 gal) |
Now run the same field on a guess. Call it 50 acres and you mix for 2.7 acres that do not exist: about 40 gallons of finished spray with 5.4 pints of product in it, sitting in the tank at the end of the day. That mix has to be applied to a labeled site or handled under the label's storage and disposal section, which varies by state.
Booms and plumbing also hold mix that never reaches the field; your sprayer manual lists the volume.
The acreage in that table is the number that decides how big the last load is, and it should come from the field, not the deed or the FSA map from 2009. The Field Boundary & Input Calculator on this site lets you draw the field boundary on aerial imagery and gives you the acres inside it. It does not do tank mix math; it gives you the true acres to feed into the table above. For odd-shaped fields, waterways and fence-row cutouts, our guide on calculating acreage for an odd-shaped field walks through the measuring side.
Both product and water scale by band width ÷ row width. A 12-inch band on 36-inch rows treats one third of the ground. A 3 lb broadcast rate at 15 GPA becomes 1 lb and 5 gallons per field acre, so a 300-gallon tank covers 60 field acres with 60 lb of product. (Ohio State FABE-530)
Keep "acres" straight here. The label rate applies to the treated band, and the tank math runs on field acres.
Herbicides usually scale with acres. Many adjuvants scale with water. A surfactant labeled as percent volume per volume (% v/v) or a conditioner labeled per 100 gallons is dosed by the gallons in the tank, however many acres it covers.
Typical label language for glyphosate, from Oklahoma State: (OSU PSS-2783)
Drop from 15 to 10 GPA and the herbicide per tank goes up while the surfactant per tank stays put. Run both through one "per tank" habit and one of them will be wrong.
A correctly calculated mix can still fail if it gels or settles out. South Dakota State's February 2026 fact sheet replaces the older W.A.L.E.S. and A.P.P.L.E.S. orders with W.A.M.L.E.G.S. (SDSU Extension P-00374)
Start by filling the tank 50 to 75 percent with clean water (or carrier such as UAN) and get agitation running. If AMS is in the mix as a water conditioner, it goes in first. Then:
Fill the rest of the way with water and keep agitating while you spray. SDSU's rule of thumb is agitation that returns 5 to 6 percent of tank volume per minute.
Jar test any combination the label does not address. Mix proportional amounts in a clear jar, in the same order, shake, and let it sit 30 to 60 minutes. Gel, sludge, flakes, an oily film or heat means the mix will do the same thing in your tank.
The label's own mixing directions override any acronym. When they disagree, follow the label.
Two different methods, and they should not be swapped.
A per-acre label rate in a backpack. Calibrate the backpack the same way (GPA), then run the same chain. A 4-gallon backpack at 40 GPA covers 0.1 acre, so it takes 10 tanks to do an acre. A 32 oz per acre rate is 3.2 oz per tank. (Cal-IPC 2023, UCCE)
A percent solution for spot spraying. Fl oz of product = percent (as a decimal) × 128 × gallons of water.
| Solution | 1 gallon | 4-gallon backpack |
|---|---|---|
| 1% | 1.28 fl oz | 5.12 fl oz |
| 2% | 2.56 fl oz | 10.24 fl oz |
The catch: a percent solution ignores how much you spray. A 2 percent mix sprayed at 20 GPA puts down 0.4 gallon of product per acre. The same mix sprayed at 100 GPA, because you are wetting brush to drip, puts down 2 gallons per acre. That can exceed the label's per-acre maximum on a patch you spot-sprayed heavily.
Percent solutions are for hand-held spot work. Never run one through a boom. Broadcast labels are written per acre.
The division is the same for every product. The limits are not. The clearest current example is over-the-top dicamba.
EPA re-registered over-the-top dicamba products (Bayer's Stryax, formerly XtendiMax; BASF's Engenia; Syngenta's Tavium) on February 6, 2026, for the 2026 and 2027 seasons. According to the label summary reported by Nebraska Extension, the labels cap use at 0.5 lb dicamba per acre per application, two applications and 1.0 lb per acre per year; require 40 oz per acre of an approved volatility reduction agent (VRA) and a drift reduction agent in every load; and prohibit AMS-containing products in the tank. (UNL CropWatch), (EPA)
As an illustration, at 20 acres per tank that VRA is 800 fl oz (6.25 gallons) per 300-gallon tank, on top of the herbicide, and the AMS a glyphosate habit puts in first is not allowed. A calculator knows none of that. The registration is being challenged in court, so check its current status before the season.
Many newer labels also send applicators to EPA's Bulletins Live! Two for location-specific endangered species limits.
A correct mix still has to land where you aimed it. The Spray Window Planner flags wind, drift and temperature inversion risk by the hour, including the dawn, dusk and calm clear-night periods when inversions set up. For the field check itself, see what wind speed is safe to spray. If you also run fertilizer by the acre, the same per-acre logic applies to nutrients in our fertilizer application rate guide.
Divide the tank size in gallons by your calibrated gallons per acre to get the acres one tank covers, then multiply those acres by the label rate per acre. A 300-gallon tank sprayed at 15 GPA covers 20 acres, so a 2 pint per acre product needs 40 pints, or 5 gallons, topped up with 295 gallons of water.
It depends entirely on your spray volume. A 300-gallon tank covers 20 acres at 15 gallons per acre, 30 acres at 10 gallons per acre, and 15 acres at 20 gallons per acre. Divide tank gallons by your calibrated GPA. If you have not caught nozzle output recently, recalibrate first, because worn tips and speed changes shift the real number.
For broadcast spraying there is no fixed per-gallon number, because labels are written per acre and the amount per gallon changes with your spray volume. For spot spraying with a percent solution, multiply the percent by 128: a 1 percent mix is 1.28 fl oz per gallon and a 2 percent mix is 2.56 fl oz per gallon.
Glyphosate jugs are sold at different strengths, from 3 to 5 lb acid equivalent per gallon. The weeds respond to the acid equivalent, not the volume of product. A 0.75 lb a.e. per acre rate takes 32 fl oz of a 3 lb product but only about 21 fl oz of a 4.5 lb product. Check the a.e. figure on every new jug.
Fill the tank 50 to 75 percent with water and start agitation, adding AMS first if it is used as a water conditioner. South Dakota State's 2026 order is W.A.M.L.E.G.S.: wettable and dry products, agitate, microencapsulated, liquids, emulsifiable concentrates, glyphosate, then surfactants. The product label's mixing directions override any general order.
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