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    DEF usage and cost calculator: how much DEF your miles use, and what it costs

    Enter the miles, your fuel economy, the DEF dosing rate as a percent of diesel and what you pay for DEF. The calculator shows the gallons, the cost, the cost per mile, the refills and how many jugs that would be.

    mi
    mpg
    % of diesel
    $/gal

    Pump price is usually lower than jugs

    $/gal
    gal
    gal

    Reference: Cummins reports DEF use of 3% to 5% of diesel fuel consumption for its X15 (checked 2026-10-10). Your engine and duty cycle may differ; your own records are best.

    DEF for 10,000 miles

    46.2 gal

    DEF cost
    $161.54
    Per mile
    $0.02
    Diesel burned
    1,538.5 gal
    Tank refills
    2
    Share of fluid cost
    2.7%
    The jug shelf: 19 jugs of 2.5 gal
    EXAMPLE rates, use yours
    Fluid cost for these miles

    Total $6,008

    $0.02 a mile in DEF on these numbers. Small costs add up; our dispatchers price from your full cost per mile, and you approve every load.

    Get loads priced on all my costs

    Diesel exhaust fluid is easy to forget because the tank is small and the bill per fill is low. Over a year of miles it adds up. This DEF usage and cost calculator works out how much DEF your miles need, using the fact that DEF use follows diesel use as a percentage. Set your own dosing rate, enter what you pay, and the jug shelf shows the volume in jugs, a picture that makes the case for buying at the pump.

    How it is calculated

    1. Diesel gallons = miles ÷ miles per gallon.
    2. DEF gallons = diesel gallons × dosing rate (as a percent).
    3. DEF cost = DEF gallons × your DEF price.
    4. DEF cost per mile = DEF cost ÷ miles.
    5. Tank refills = DEF gallons ÷ DEF tank size. Jugs = DEF gallons ÷ jug size, rounded up.
    6. Share of fluid cost = DEF cost ÷ (diesel cost + DEF cost).

    Where the dosing rate comes from

    The rate is an input because engines differ. As a reference, Cummins reports DEF use of 3% to 5% of diesel fuel consumption for its X15, and notes that higher fuel use and long idle times raise it. Check your engine maker's figure, or better, record every DEF fill against your fuel for a month and divide.

    Source: Cummins newsroom, 'A broader view of efficiency drives the Cummins X15' (May 18, 2026), checked October 2026

    A worked example

    10,000 miles of DEFExample

    1. DIESEL10,000 mi ÷ 6.5 mpg = 1,538.5 gal
    2. DEF1,538.5 × 3% = 46.2 gal
    3. COST46.2 gal × $3.50 = $161.54, about $0.016 per mile
    4. REFILLS23-gal tank: about 2 refills
    5. JUGSOr 19 jugs of 2.5 gal

    These are EXAMPLE rates and prices. Your dosing rate and the price you pay will differ.

    How to read the result

    The cost per mile is small next to diesel, which is why it's easy to leave out of your numbers. Put it in anyway: a cent or two per mile across a year's miles is real money, and the share of fluid cost shows how it compares with fuel. If your measured rate is well above your engine maker's figure, idle time is the first thing to check, then the aftertreatment system itself.

    Saving on DEF

    • Buy at the pump. Pump DEF usually costs less per gallon than jugs, and it's faster.
    • Cut idling. Idling burns diesel and, with it, DEF.
    • Store it right. DEF degrades in heat and sunlight and freezes in cold; store jugs out of direct sun and keep the cap on.
    • Keep it clean. Contaminated DEF can damage the aftertreatment system. Use a dedicated funnel or the pump nozzle, never a fuel funnel.
    • Track it. Log every DEF fill with the fuel fill. A rising rate is an early warning.

    Pump vs jug: run both prices

    Put your jug price into the calculator, note the cost, then put in the pump price you see on your route. The difference, times the miles you run in a year, is what buying at the pump is worth to you. For a truck that runs a lot of miles, it can pay for a planned stop at a station with DEF at the island. Keep one sealed jug aboard for routes where pumps are scarce, and buy the rest at the pump.

    DEF in your cost per mile

    DEF belongs with fuel in your variable costs. Add it to the trip's fuel with the trip fuel cost calculator, and in winter, check the anti-gel additive dosing calculator for the other fluid that goes into the tank. To see what a dispatch fee costs alongside your running costs, use the dispatcher fee calculator. For how running costs show up in freight prices, see freight shipping cost (coming soon) and our freight cost analysis (coming soon).

    Every cent per mile matters when you counter on a rate. Our step deck dispatch team, like every desk here, prices from your full cost per mile, and you approve every load. Send an application.

    Questions about this calculator

    How much DEF does a truck use?
    DEF use follows diesel use, as a percentage of the fuel burned. Cummins, for example, reports 3 to 5 percent of diesel consumption for its X15 engine, varying with duty cycle. Heavy loads, hills and long idling raise it. Divide your miles by your mpg for diesel gallons, then apply your engine's rate.
    How much DEF per gallon of diesel?
    At a 3 percent dosing rate, about 0.03 gallons of DEF for each gallon of diesel, or 3 gallons of DEF per 100 gallons of diesel. At 5 percent, it's 5 gallons per 100. The rate depends on the engine and how it's driven, so track your own DEF fills against fuel for a few weeks to find your number.
    How much is DEF per gallon?
    It varies by where and how you buy it. DEF from a pump at a truck stop usually costs less per gallon than DEF in jugs or boxes from a store, and bulk buyers often pay less again. Prices also move with urea and natural gas markets. Enter the price you actually pay in the calculator.
    What is DEF cost per gallon of diesel burned?
    Multiply your dosing rate by the DEF price. At a 3 percent rate and $3.50 a gallon of DEF, each gallon of diesel needs about $0.105 of DEF. Divide by your mpg for a cost per mile. It's a small number on one load and a real line in a year's budget.
    What happens if a truck runs out of DEF?
    Engines with selective catalytic reduction need DEF to meet emissions standards. When the DEF level gets low, the truck warns the driver, and running out usually brings power or speed limits until the tank is refilled. Your owner's manual describes exactly what your engine does. Keep the tank topped up and carry a jug if your route has few pumps.