Spray Tank Refill Calculator
Enter your tank size, leftover mix and product rate to find how much concentrate to add when topping off a partially full spray tank. No signup.
How this is calculated
The math follows the OSU FABE-530 refill method: when the tank is not empty, substitute the refill volume for the full tank capacity in every calculation. The leftover mix already carries its original concentration, so only the fresh water needs product.
refill volume = tank capacity - leftover mix
product to add = refill volume x product rate (oz/gal)
water to add = refill volume - (product to add / 128)Worked example (from FABE-530): a 300-gallon tank with 75 gallons of spray left. Refill volume = 300 - 75 = 225 gallons. The label calls for 2 pints per acre (= 32 fl oz per acre). At a calibrated 15 GPA, the product rate is 32 / 15 = 2.133 oz/gal. Product to add = 225 x 2.133 = 480 fl oz (30 pints). Actual water to add = 225 - (480 / 128) = 221.25 gallons.
The 128 constant is fluid ounces per US gallon. The
product rate (oz/gal) comes from the label when it states oz per
gallon, or from the label rate divided by your calibrated GPA. The
sprayer
calibration calculator gives your calibrated GPA (test strip or
boom formula), or use the
herbicide
tank mix calculator to convert an oz-per-acre label rate into
oz/gal via your GPA. The
pesticide
dilution calculator handles the same conversion for percent and
ratio label formats.
Source: Ohio State University Extension, FABE-530, "How Much Chemical Product Do I Need to Add to My Sprayer Tank?" by Erdal Ozkan (retrieved 2026-10-18).
Sources
- How Much Chemical Product Do I Need to Add to My Sprayer Tank? (FABE-530). Ohio State University Extension. Retrieved .