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DEF Cost Calculator for Owner Operators

See how much diesel exhaust fluid your truck uses and what it costs per mile and per year, and work out your own DEF rate from your fill-ups.

Your truck

% of diesel
$/gal
$/gal
gal
gal
$/gal

Prices and tank sizes are examples. Change every number to match your truck and what you pay.

What DEF costs you

DEF cost per mile

$0.0138

$13.85 per 1,000 miles

DEF cost a year

$1,385

461.5 gallons, 23.1 tanks

Miles per gallon of DEF

216.7

4.6 gallons per 1,000 miles

One DEF tank lasts

4,333 mi

6 gal of DEF per full diesel tank (1,300 mi)

DEF adds up to 1.5% of what you spend on diesel.

Find your own DEF rate

Add up the DEF you put in and the diesel you burned over the same stretch, like a month. Fill both tanks at the start and the end so the count is fair.

gal
gal
To add DEF to your full cost of running, use the Cost Per Mile Calculator. To plan fuel stops, see the Fuel Range Planner. If a DEF warning comes on, check the Fault Code Lookup.

DEF gallons are diesel gallons times the DEF rate. Diesel gallons are miles divided by MPG. Cost is DEF gallons times the DEF price. Heavy Duty Trucking has reported manufacturers predicting DEF rates of 2 to 3 percent for over-the-road trucks, but the real rate depends on your engine, load and driving, so use your own numbers. Your numbers stay in this browser. This is an estimate for planning.

Numbers tight? Let a dispatcher find better loads.

This tool is free. If it shows you are leaving money on the road, Grow Trucking finds loads, negotiates rates and does the paperwork for owner-operators and small fleets.

Talk to a dispatcher Call (845) 400-9179

Also: About Grow Trucking · Refer a carrier · Trucking blog

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How much does DEF cost a semi truck?

Heavy Duty Trucking reports manufacturers predicting DEF use of 2 to 3 gallons for every 100 gallons of diesel. At 6.5 MPG and 3 percent, DEF costs about 1.4 cents a mile at $3.00 a gallon, roughly 2 percent of the diesel bill. Use your own rate, since it depends on the engine and the work.

  1. Enter your diesel MPG and the price you pay for DEF and diesel.
  2. Enter your DEF rate, or work it out from your own fill-ups in the box below.
  3. Enter miles a year and your tank sizes.
  4. Read DEF cost per mile, per year, and how far a DEF tank goes.
  5. Enter a second DEF price to see what a cheaper source would save.
  6. Add the per-mile cost to your running cost per mile.
How the DEF numbers are worked out
NumberHow it is worked out
Diesel gallonsMiles / diesel MPG
DEF gallonsDiesel gallons x DEF rate
DEF costDEF gallons x DEF price
Miles per gallon of DEFDiesel MPG / DEF rate
One DEF tank lastsTank gallons x miles per gallon of DEF
Your own DEF rateDEF gallons added / diesel gallons burned x 100

The 2 to 3 percent figure is what manufacturers were reported to predict for over-the-road trucks in Heavy Duty Trucking (August 25, 2011). Real rates vary with the engine, load and duty cycle, so the tool lets you enter your own. Prices and tank sizes on the page are examples. EPA's DEF guidance letters are dated August 11, 2025 and March 26, 2026.

Last reviewed September 2026. Sources: Heavy Duty Trucking: Don't forget to factor DEF into fuel costs; EPA: diesel exhaust fluid and DEF guidance; EIA: weekly U.S. on-highway diesel prices.

Frequently Asked Questions - DEF Cost Calculator

How much DEF does a semi use?
Heavy Duty Trucking has reported that manufacturers predict DEF rates of 2 to 3 percent for over-the-road trucks. That is 2 to 3 gallons of DEF for every 100 gallons of diesel. At 6.5 MPG and 3 percent, a truck uses about 4.6 gallons of DEF per 1,000 miles. Your engine, load and driving change the real number.
How much does DEF cost per mile?
Multiply the diesel gallons per mile by your DEF rate and your DEF price. At 6.5 MPG, a 3 percent rate and $3.00 a gallon, DEF costs about 1.4 cents a mile, or about $1,385 a year over 100,000 miles. The calculator does this with your own numbers.
How many miles per gallon of DEF does a truck get?
Divide your diesel MPG by your DEF rate. At 6.5 MPG and 3 percent, that is about 217 miles per gallon of DEF. At 8 MPG and 2 percent it is 400. The calculator shows this and how far one DEF tank goes.
How often do I fill the DEF tank on a semi?
It depends on the tank size and your DEF rate. A 20 gallon tank at 6.5 MPG and 3 percent goes about 4,300 miles, so a truck that drives 100,000 miles a year fills it about 23 times. Enter your own tank size to see yours. A common habit is to top it off when you fuel.
How do I find my real DEF rate?
Fill both tanks, then note the DEF gallons you add and the diesel gallons you burn over a month, and fill both again at the end. Divide the DEF gallons by the diesel gallons and multiply by 100. The Find your own DEF rate box does this and can put the answer into the calculator.
Is DEF worth the cost?
SCR systems that use DEF let the engine run for better fuel economy. Heavy Duty Trucking said the way to check is to take the DEF rate away from the fuel economy gain. If a truck gets 8 percent better fuel economy and uses 3 percent DEF, the net gain is 5 percent. You need it to run an SCR truck either way, so the question is what it costs.
What happens if a semi runs out of DEF?
The truck warns you first. If a DEF problem is not fixed, many trucks eventually limit power or speed, which is called a derate. When and how depends on the engine maker and model year. EPA issued DEF guidance letters in August 2025 and March 2026 asking makers to ease derates and allow other sensors. Ask your dealer what your truck does.
Is DEF cheaper in bulk?
Prices differ by where and how you buy, such as at the pump, in a jug or in bulk. Enter a second price in the calculator to see the yearly difference. Weigh it against storage, handling and keeping the fluid clean.
Where does DEF go in my cost per mile?
Put it in your running cost per mile, the part that does not include fuel. The load tools on this site work out fuel separately from your MPG, so adding DEF to your cost per mile without fuel keeps it counted once.

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Go Further With DEF Cost Calculator

Guides, services and reference pages that go with this tool.