Carpool Savings Calculator

Type your starting postcode (e.g. home) and destination postcode (e.g. work) into the Route Details card. The calculator uses these to determine the driving distance and select regional fuel prices. If both postcodes are in the same region, you get the local fuel price. If they span two regions, prices are blended.

The distance comes from a lookup table of common UK inter-regional routes (e.g. London to Manchester = 211 miles). For routes not in the table, the calculator uses the straight-line distance multiplied by a 1.21 road-winding factor, based on DfT travel survey data.

Choose your fuel type: petrol, diesel, or electric. Each type uses different efficiency and emissions assumptions. You can optionally enter your actual MPG (petrol/diesel) or miles per kWh (electric) — otherwise the calculator uses UK fleet-average values from the Department for Transport.

The registration field is a label for your own reference only — no DVLA lookup is performed. If you want to find your car's actual specifications, check your V5C document or the manufacturer's data sheet.

Set the total number of people sharing the car (2 to 6, including the driver) and how many trips you make per week. Then choose how to split costs:

• Equal split — everyone pays the same share of the fuel bill
• Driver Discount — the driver pays nothing, passengers split the full cost (common for school runs or neighbourhood shares)
• Custom — set the driver's percentage (0-50%), passengers split the rest equally

Finally, choose your planning horizon: weekly gives you the per-week figure, monthly is most useful for budgeting, and yearly shows the full annual saving.

After clicking Calculate Savings, you'll see four result cards:

• Headline savings — the total group saving for your chosen period
• Financial breakdown — solo vs carpool cost comparison plus per-person split
• Annual projection — what the arrangement saves over a full year, with real-world equivalents (cinema tickets, coffees, food shops)
• Environmental impact — CO₂ reduction in kilograms, plus tree-planting equivalents

Use the Export CSV button to download the full breakdown. This is useful for employer reimbursement claims or personal budgeting.

The calculator uses a two-tier approach to estimate driving distance:

1. Lookup table: For common inter-regional routes (e.g. London↔Manchester at 211 miles, London↔West Midlands at 126 miles), the calculator uses a pre-measured road distance from DfT data.
2. Haversine fallback: For routes not in the lookup table, it calculates the straight-line (great-circle) distance between your two postcode area centres and multiplies by 1.21 — the average UK road-winding factor derived from comparing straight-line vs actual road distances across hundreds of UK routes.

This gives a reasonable estimate for most commutes. For very precise mileage, use the Fuel Cost Trip Calculator where you can enter the exact distance from your satnav or Google Maps.

The environmental calculation works differently depending on fuel type:

• Petrol/Diesel: CO₂ is calculated as distance (km) × the vehicle's CO₂ emissions per km. These figures come from EU type-approval fleet averages: approximately 170 g/km for petrol and 160 g/km for diesel.
• Electric: CO₂ is calculated as electricity consumed (kWh) × your region's grid carbon intensity (g CO₂/kWh). Scotland has a significantly cleaner grid (~74 g/kWh due to wind and hydro) compared to England (~150 g/kWh). This means an EV carpool in Scotland produces much less CO₂ than the same journey in London.

The "solo" scenario assumes every person drives separately. The "carpool" scenario assumes one car carries everyone. The CO₂ saving is the difference — fewer cars on the road means proportionally less total emissions.

Carpooling isn't always the cheapest option. Consider these scenarios:

• Very short trips: If your commute is under 3 miles, walking or cycling is free and faster in many urban areas.
• Public transport zones: In London and other cities with good bus/rail networks, a monthly travel card may be cheaper than shared fuel costs — especially for a single passenger.
• Detour penalty: If picking up passengers adds significant extra mileage, the fuel cost of the detour can eat into savings. This calculator doesn't model detours — it assumes all passengers share the same start-to-end route.

For public transport comparisons, try the UK Commute Cost Calculator which covers train, bus, and cycling alternatives.