The basic formula
Daily fuel cost = (round-trip miles ÷ MPG) × gas price. Multiply by working days (~250/year) for annual fuel. For 50 miles round-trip at 28 MPG and $3.45/gal: $6.16/day × 250 = $1,540/year in fuel alone.
Tools
Use the Cost Per Mile Calculator for total cost, and the Fuel Cost Calculator for daily gas.