Record Fuel Purchases and Calculate Consumption Between Full Fill-Ups

The short answer

To calculate fuel consumption from your own records, you need two full fill-ups of the same vehicle. A full fill-up means filling the tank completely, not just adding a set amount or dollar value. The distance driven is the ending odometer reading minus the starting odometer reading. The fuel used is everything you bought after the starting full fill, up to and including the ending full fill. Divide one by the other in the unit you prefer: miles per gallon (MPG), kilometers per liter (km/L) or liters per 100 kilometers (L/100 km).

Two conditions change the result. First, partial fills between the two full fills are not discarded. They are added into the same interval. Second, if any purchase in the interval is missing, or you cannot tell whether a boundary fill was full, that interval cannot be calculated reliably. In that case, wait for the next complete full-to-full interval.

What to record at every fuel purchase

Record every purchase, including small top-ups. One missing receipt breaks the interval it belongs to. A record kept in a notebook, a spreadsheet or an app should capture the same core details.

Field	What to enter	Why it matters
Vehicle	Plate, nickname or other reference	Keeps records from different vehicles separate
Date (and time if useful)	When you bought the fuel	Puts purchases in the correct order
Odometer reading	Total distance shown at the pump, with unit (mi or km)	Defines the distance of each interval
Fuel quantity and unit	Amount from the pump display or receipt, in gallons or liters	The fuel side of the calculation
Fill status	Full, partial or unknown	Determines which records can start or end an interval
Total cost and currency	Amount paid	For spending records, kept apart from consumption
Receipt reference	Receipt number or photo file name	Lets you trace or correct an entry later
Notes	Station, trip type, anything unusual	Helps explain an odd result

Fuel-logging apps use a similar structure. The FillUp app, for example, logs purchase date and time, odometer, quantity in gallons or liters, whether the tank was filled, total fuel cost and notes. Its CSV export also includes calculated MPG, km/L or L/100 km. If you set up a spreadsheet yourself, keep one row per purchase. If a value is missing, leave the cell blank rather than guessing, so a gap stays visible.

Vehicle  Date        Odometer  Unit  Fuel   Unit  Fill     Cost   Curr  Receipt
Car-A    2026-03-02  12000     km    38.0   L     full     ...    ...   R-0101
Car-A    2026-03-09  12260     km    15.0   L     partial  ...    ...   R-0102
Car-A    2026-03-15  12500     km    25.0   L     full     ...    ...   R-0103

How to find a complete full-to-full interval

Before you calculate, check that the run of records between two full fills can be trusted:

All entries belong to the same vehicle.
Odometer readings increase from one entry to the next.
Distance units are consistent throughout, and fuel units are consistent throughout.
Both the starting and ending records are marked full, not partial or unknown.
No purchases are missing between the two full fills.
Note the receipt references of the starting and ending full fills so the result can be traced back to its records.

If any check fails, mark that interval as unresolved. Do not estimate the missing fuel. Your next pair of consecutive full fills with a complete record in between gives a fresh, usable interval.

Calculation steps

1. Distance: subtract the starting full-fill odometer from the ending full-fill odometer.
2. Fuel: add up every purchase after the starting full fill, through and including the ending full fill. Do not count the fuel bought at the starting fill, because it refilled what was used before the interval began.
3. Combine partial fills: any partial purchases in between go into the same fuel total.
4. Choose your unit:
MPG = miles ÷ gallons
km/L = kilometers ÷ liters
L/100 km = liters ÷ kilometers × 100

Fuelio describes the same full-tank method. It needs at least two full fill-ups, and it pairs the distance between them with the fuel added after the first. Partial purchases in between count toward that interval. They do not produce results of their own. Fuelio can also estimate consumption from tank levels and a tank capacity that you enter, but that is a separate kind of estimate, not a full-to-full calculation.

Worked examples (hypothetical figures)

The figures below are made up to show the arithmetic. They do not describe any real vehicle's performance.

Metric example: a full fill at 12,000 km starts the interval. Later you buy a partial 15 L. You then fill to full at 12,500 km, adding 25 L. No purchases are missing. Distance = 12,500 − 12,000 = 500 km. Interval fuel = 15 + 25 = 40 L; the fuel bought at the 12,000 km fill is not counted. Consumption = 40 ÷ 500 × 100 = 8 L/100 km. Distance per liter = 500 ÷ 40 = 12.5 km/L.

US example: a full fill at 30,000 miles starts the interval. A partial fill of 4.0 gallons follows. You then fill to full at 30,300 miles, adding 6.0 gallons. Distance = 300 miles. Fuel = 4.0 + 6.0 = 10.0 gallons. Result = 300 ÷ 10.0 = 30 MPG.

Keeping units consistent

Decide on one distance unit and one fuel unit before you calculate. Convert any mixed records first. Label which gallon you are using. A US gallon is about 3.785 liters, while an imperial gallon, used in the UK, is about 4.546 liters. The same distance and fuel therefore give different MPG figures depending on which gallon is meant. To compare a US MPG figure with L/100 km, divide about 235.2 by the MPG value. For example, 30 US MPG is roughly 7.8 L/100 km.

Keep cost totals separate from consumption

Fuel cost belongs in the same record, but keep it out of the consumption calculation. Price changes between fills say nothing about how much fuel the vehicle used. Total the costs on their own for budgeting. If you want cost per mile or per kilometer, use the cost of exactly the same purchases that made up the interval. That keeps the spending figure tied to the same records as the consumption result.

Each result describes past driving over that one interval only. Logging every purchase consistently over many intervals gives you a traceable history. You can then compare the vehicle against itself over time.