Fleet fuel carbon calculator (diesel and petrol)

How this calculation works

The formula is the simplest in carbon accounting, and also one of the most defensible:

Emissions (kgCO₂e) = litres of fuel burned × combustion factor (kgCO₂e/litre)

Every litre of diesel or petrol that goes through an engine turns into a fixed, well-understood quantity of CO₂, plus small amounts of methane and nitrous oxide. The conversion factor bundles all three into a single CO₂-equivalent number per litre. Because combustion chemistry doesn't care whether the engine is in a delivery van in Tuas or a lorry in Manchester, fuel factors travel well across borders — which is why the UK's DEFRA/DESNZ factors are the standard international default for fleets.

Fuel burned in vehicles your organization owns or leases is Scope 1 — direct emissions from sources under your control. For a services or logistics organization with no factory and no gas boiler, the fleet is usually the whole of Scope 1, which makes this the first number to pin down after electricity.

Why litres beat mileage

There are two ways to estimate fleet emissions: fuel-based (litres × factor) and distance-based (kilometres × an assumed consumption rate × the same factor). This calculator deliberately uses the fuel-based method, because the distance-based one smuggles in a guess.

Manufacturer fuel-economy figures are measured under test conditions. Real-world consumption moves with payload, air-conditioning, stop-start urban traffic, tyre pressure, and how heavy your drivers' right feet are. A van rated at one consumption figure can burn meaningfully more in daily city work, and a distance-based estimate inherits every bit of that error. Litres purchased, by contrast, are invoice-grade primary data: you paid for them, the receipt exists, and nobody can argue with a metered pump.

The best data source is the fuel-card statement. Most fleet fuel cards (corporate cards from the major station networks) issue a monthly statement listing litres per transaction, per card, per vehicle. One document gives you the complete monthly input for this calculator plus an audit trail back to individual fills. If you don't run fuel cards, pump receipts and bulk-tank purchase records do the same job with more collation effort.

The factors we apply

| Fuel | Factor (kgCO₂e/litre) | Source | | --- | --- | --- | | Diesel | 2.686 | DEFRA/DESNZ 2025 | | Petrol | 2.3153 | DEFRA/DESNZ 2025 |

For reference, the same 2025 factor set puts LPG at 1.5594 and CNG at 2.54 kgCO₂e per litre — if part of your fleet runs on those fuels, the result here will be incomplete and you should track them separately. The DEFRA factors are published annually under the Open Government Licence v3.0, which is what allows us to serve them free with attribution. The exact factor, data year, and licence are shown under your result, so if a customer or consultant ever challenges the number, the provenance is already attached.

A worked example

Say you run a small delivery fleet: six diesel vans and two petrol cars. The June fuel-card statement shows 500 litres of diesel and 200 litres of petrol.

500 L diesel × 2.686 kgCO₂e/L = 1,343.0 kgCO₂e

200 L petrol × 2.3153 kgCO₂e/L = 463.1 kgCO₂e

Total = 1,806.1 kgCO₂e ≈ 1.81 tCO₂e for the month

At a steady pace that is roughly 21.7 tCO₂e a year — for many organizations, comfortably the largest single line in the footprint, bigger than the office electricity it sits next to. Doing this monthly from the fuel-card statement takes minutes and builds the twelve-month record a reporting framework will eventually ask for.

When the fleet isn't just vehicles: generators on site

Construction firms, facilities managers, and any organization running events or works on sites without a stable grid connection often have a second Scope 1 fuel line that has nothing to do with driving: diesel generators providing power to a site office, tower crane, or temporary works. That diesel goes through an engine and is metered the same way as vehicle fuel — by the litre, off a fuel-card statement or bulk-delivery docket — and it carries the identical 2.686 kgCO₂e/litre factor. The practical difference is documentation: generator fuel is often delivered in bulk to a site rather than drawn per-vehicle from a fuel card, so the evidence trail is a delivery docket rather than a card statement. Keep generator litres as their own line within Scope 1 rather than folding them into "vehicles" — a reviewer comparing your fuel number to your vehicle count will otherwise find the arithmetic doesn't add up.

A second worked example: adding a generator to the fleet total

Extend the delivery-fleet example above with a site generator that burned 300 litres of diesel during the same month, run to power a temporary warehouse extension while mains power was being connected.

Vehicles (from above): 1,806.1 kgCO₂e

Generator: 300 L × 2.686 kgCO₂e/L = 805.8 kgCO₂e

Combined Scope 1 fuel: 2,611.9 kgCO₂e for the month

The generator alone adds close to half again on top of the eight-vehicle fleet's total — a reminder that "fleet fuel" and "Scope 1 fuel" are not always the same scope of work, and that a site running temporary power for even a few weeks a year can materially move the annual number.

Common mistakes with fleet fuel accounting

• Blending personal reimbursement claims into fleet fuel. An employee's own car, refuelled on a business trip and claimed back on expenses, is grey-fleet Scope 3, not Scope 1 — mixing that mileage reimbursement into the fuel-card total mis-scopes it and usually understates the correct Scope 3 travel figure elsewhere.
• Estimating litres from odometer readings instead of using card data. Where fuel-card statements exist, an odometer-and-assumed- economy estimate is strictly worse data that this calculator's method was built to avoid — use the card litres if you have them.
• Forgetting off-road and generator diesel entirely. Vehicle fuel cards capture driving, but bulk diesel delivered straight to a site generator or piece of plant equipment often runs through a completely separate purchasing process and gets missed unless someone specifically asks facilities or site management for those delivery dockets.
• Applying the standard factor to a biofuel blend without checking. A fleet quietly switched to B20 or a similar blend by its fuel supplier will be overstated if the standard 2.686 diesel factor is applied to the whole volume — confirm the blend percentage before assuming ordinary forecourt diesel.

What this calculator doesn't cover

Being clear about the edges is part of the method:

• Biofuel blends. The diesel and petrol factors represent ordinary forecourt fuel. DEFRA publishes a separate biodiesel factor of just 0.1694 kgCO₂e/litre, because under GHG Protocol rules the biogenic CO₂ from the bio-derived share is reported outside the scopes rather than in Scope 1. If your fleet runs B20, B100, or renewable diesel, applying the standard 2.686 will overstate your Scope 1 — get the blend percentage from your supplier and split the litres before calculating.
• Well-to-tank emissions. Refining and transporting the fuel before it reached your tank is a real emissions burden, but it belongs in Scope 3 (fuel- and energy-related activities), not here.
• Grey fleet and hired transport. Employee-owned cars on business trips, taxis, and subcontracted couriers are Scope 3. Subcontracted freight is normally estimated per tonne-kilometre (road freight is 0.1153 kgCO₂e per tonne-km in the same DEFRA set) — a different method entirely.
• Electric vehicles. EV charging is purchased electricity, which is Scope 2 — put those kWh through the business electricity calculator instead.
• Vehicle air-conditioning refrigerant leaks. Technically Scope 1 fugitive emissions, but they need refrigerant top-up records, not fuel data.

None of these gaps make the fuel number wrong; they define what it is — your direct combustion emissions from owned and leased vehicles, and nothing else.

Where this fits in your reporting

Fuel burned in vehicles you own or lease is Scope 1 under the GHG Protocol's Corporate Standard — a direct emissions source, not an indirect one. That figure feeds any framework that asks for Scope 1: ISSB IFRS S2, GRI 305-1, a CDP questionnaire, or a national reporting code. The arithmetic — litres times factor — is identical whether the fleet belongs to a logistics company, a nonprofit's outreach vans, a school's bus route, or a public agency's works vehicles. For Singapore-listed filers working to a fixed timetable, see the SGX reporting deadlines guide; everyone else needs the same litres and the same two factors.

When you'd rather not type the litres in by hand, upload the fuel-card statement itself — the same factors get applied to figures read straight off the PDF, with a confidence score on every extracted field.

Sources

• UK Government (DESNZ/DEFRA), Greenhouse gas reporting: conversion factors — annual company-reporting factor sets (Open Government Licence v3.0): gov.uk/government/collections/government-conversion-factors-for-company-reporting
• GHG Protocol, Corporate Accounting and Reporting Standard — Scope 1 definition and biogenic CO₂ treatment: ghgprotocol.org/corporate-standard