Living together
How to split the cost of a shared car
Gas money is the part everybody remembers and it is under a quarter of what a mile costs. Once depreciation, insurance and servicing are in, the owner of a shared car is usually donating about 39 cents of every mile somebody else drives. Here is the rate that fixes it.
Updated September 20268 min read
The short version
Add up everything the car costs in a year, including what it loses in value, then divide by the miles it does. That gives you a cost per mile. Everyone pays for the miles they drove.
The number is always larger than people expect, because fuel is the only cost anybody sees weekly. Depreciation is the biggest single item and it is invisible until the day the car is sold.
The number
Why gas money is not the answer
Fuel is the only car cost with a receipt attached to a specific trip, so it is the one everybody offers to share. Everything else arrives annually, or once, or at the end, and none of it feels caused by the drive to the airport last Tuesday.
All of it is caused by driving, though. Servicing intervals are measured in miles. Tires wear by the mile. Most of what a car loses in value it loses because it accumulated miles and years, and the miles part is the part a borrower is consuming.
Filling the tank feels like paying for the car. It covers about a quarter of what the trip actually cost.
This is why the arrangement always ends the same way. It works for months, the owner feels vaguely resentful without being able to say why, and then a $900 service bill arrives and one person pays it. The bill is not the problem: the rate was.
The worked example
Three roommates share one car. Ada does most of the driving, Cara barely uses it.
- Depreciation: $18,000 down to $7,000 over five years, so $2,200 a year
- Insurance $1,400, servicing and tires $900, tax and registration $200
- Fuel: 12,000 miles at 30 mpg is 400 gallons, at $3.60 that is $1,440
- Total $6,140 a year, of which $4,700 is standing cost that exists whether the car moves or not
Ada
- Miles
- 6,000
- Real share
- $3,070.00
- Fuel only
- $720.00
- What the owner absorbs
- $2,350.00
Ben
- Miles
- 4,000
- Real share
- $2,046.67
- Fuel only
- $480.00
- What the owner absorbs
- $1,566.67
Cara
- Miles
- 2,000
- Real share
- $1,023.33
- Fuel only
- $240.00
- What the owner absorbs
- $783.33
| Driver | Miles | Real share | Fuel only | What the owner absorbs |
|---|---|---|---|---|
| Ada | 6,000 | $3,070.00 | $720.00 | $2,350.00 |
| Ben | 4,000 | $2,046.67 | $480.00 | $1,566.67 |
| Cara | 2,000 | $1,023.33 | $240.00 | $783.33 |
Read the last column as a loan nobody agreed to. If the car belongs to Ada and the other two pay fuel only, she is carrying $2,350 of their driving in a single year. A month of ordinary use, 500 miles, costs $255.83 at the real rate and $60.00 in fuel.
Depreciation is not a fee you invent
Work out the rate for your own car
The answer depends completely on the car, which is why a single quoted rate is useless: an old paid-off hatchback and a new financed estate are not within twenty cents a mile of each other. Put in your own numbers.
The car
Every year
Who drives it, and how far in a year
What a mile really costs
51.2 cents
$6,140.00 a year over 12,000 miles
Fuel only
12.0 cents
$1,440.00 a year, so 39.2 cents a mile lands on the owner
| Driver | Miles a year | Their real share | If they only paid fuel |
|---|---|---|---|
| Ada | 6,000 | $3,070.00 | $720.00-$2,350.00 |
| Ben | 4,000 | $2,046.67 | $480.00-$1,566.67 |
| Cara | 2,000 | $1,023.33 | $240.00-$783.33 |
The real-share column adds back to $6,140.00 exactly. Nothing you type here leaves your browser.
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Two different arrangements, two different rates
A genuinely shared car
Nobody would own it alone, everybody uses it, and it exists because the household needs a car. Then the all-in rate is right and the standing costs are shared like any other household bill: the car is a joint asset and the miles decide who pays what.
Borrowing somebody else’s car
The owner would have the car regardless, and a roommate uses it occasionally. Charging them a share of insurance the owner was always going to buy is overcharging. The fair rate here is fuel plus a per-mile contribution to the costs their driving actually causes: servicing, tires, and the mileage part of depreciation. In the example that is a few tens of cents a mile rather than 51.2, and far more than 12.
Deciding which situation you are in is the whole conversation, and it is worth having explicitly rather than discovering the disagreement when a bill lands. How to split shared costs fairly covers choosing a method for that kind of decision, and splitting utility bills when usage isn’t equal is the same shape of argument applied to a bill that arrives monthly.
Tickets, damage, and the bit that is not arithmetic
Tickets and fines belong to the driver
Always, without discussion, including the ones that arrive by mail three weeks later. Agree it before the first one, because agreeing it afterwards sounds like an accusation.
Damage is not a running cost
Ordinary wear belongs in the shared pot. A specific incident belongs to whoever was driving, including the insurance deductible, and this is the one place where a written note beats a conversation everybody remembers differently.
Insurance is a legal question before it is a money one
Whether a roommate is covered to drive your car depends on your policy and your jurisdiction, and getting it wrong is expensive in a way no split can fix. Check the policy wording before the money conversation, not after.
Read the odometer monthly
One number, on the first of the month, written down where everybody can see it. Per-trip logging collapses within a month in every household that tries it, and a monthly reading is accurate enough for a rate that is itself an estimate.
Common questions
How much should you charge someone for using your car?
Is gas money enough?
Should depreciation really be included?
What if the owner would have kept the car anyway?
How do you track the miles without it becoming a chore?
Who pays for a repair, or for the parking ticket?
Or track the miles and the money in one place
Halvy handles weighted splits like miles driven, keeps recurring costs such as insurance on a schedule, and holds one running total so the annual bills are not a surprise anybody has to absorb. Free on iOS and Android, no ads, and it never asks to connect to a bank.
Every figure here was produced by running Halvy’s own split engine on the worked example, which is why the per-person shares reconcile to $6,140.00 exactly. The car’s costs are an example rather than a survey: prices, insurance and resale values vary enormously, which is what the calculator above is for. Whether another driver is insured on your policy is a matter for your insurer and your jurisdiction rather than for arithmetic.