How the Lease vs. Buy Math Works

The lease side: three ingredients

A car lease payment looks like dealer magic, but it's three simple pieces added together. Once you can see them, you can check any quote yourself.

1. The depreciation fee — paying for the part of the car you use up

When you lease, you're renting the slice of the car's value that disappears while you drive it. The leasing company predicts what the car will be worth at the end — the residual value — and you pay the difference, spread evenly over the term:

depreciation fee = (capitalized cost − residual value) ÷ term in months

The capitalized cost ("cap cost") is just the negotiated price of the car, minus anything you put down (your "cap cost reduction"). Yes — the price is negotiable on a lease, exactly as it is on a purchase, and every dollar you negotiate off drops straight into this formula.

The residual is set by the leasing company, usually quoted as a percentage. A car that holds its value well has a high residual, which shrinks the depreciation fee — this is why the same monthly budget leases "more car" in some brands than others.

2. The rent charge — the interest, wearing a disguise

The money factor is the lease world's way of writing an interest rate. Multiply it by 2,400 to get the roughly equivalent APR: a money factor of 0.00125 is about a 3% rate. Dealers quote it as a tiny decimal partly because tiny decimals don't invite negotiation — but it is negotiable, and it must be disclosed if you ask.

rent charge = (capitalized cost + residual value) × money factor

Why cap cost plus residual? It's a shortcut for charging interest on the average amount of the car's value you're effectively borrowing over the term. The sum-times-money-factor form bakes the averaging (and the ÷2) into the money factor itself — which is exactly why the APR conversion is ÷2,400 rather than ÷1,200.

3. Sales tax

Most US states tax a lease the friendly way: on each monthly payment, not on the whole car.

monthly lease payment = (depreciation fee + rent charge) × (1 + tax rate)

A handful of states (for example Texas, and Illinois before 2015's reform in the other direction — check your own state's current rule) tax the full vehicle price up front instead, which meaningfully worsens the lease side. The calculator uses the monthly method; if your state taxes differently, treat its lease total as optimistic and check your quote's tax line.

The lease total

lease total = down payment + (monthly lease payment × term)

At the end you hand back the keys and own nothing — which is not a hidden gotcha, it's the deal. The buy side's math is different for exactly that reason.

The buy side: a loan, then an asset

A car loan is standard amortization — the same formula as a mortgage. Sales tax on the full price is rolled into the amount financed:

amount financed = price × (1 + tax rate) − down payment

monthly payment = L × r × (1+r)ⁿ ÷ ((1+r)ⁿ − 1)

where L is the amount financed, r is the monthly rate (APR ÷ 12), and n is the loan term in months. (At 0% APR this collapses to simply L ÷ n.)

To compare fairly against the lease, the calculator stops the clock at the end of the lease term and asks: where does the buyer stand? Three numbers answer that:

• Payments made so far — the monthly payment times the months elapsed.
• Loan balance still owed — because car loans often run 60 or 72 months while leases run 36, the buyer may still owe money at the horizon: balance = L(1+r)^h − M((1+r)^h − 1)÷r after h payments of M.
• The car itself — your estimate of its resale value at the horizon, counted as money back, because the buyer can sell it (or keep driving it payment-free, which is where buying quietly wins over longer horizons).

buy total = down payment + payments made + balance still owed − expected resale value

The optional opportunity-cost line

If buying takes $4,000 down and leasing takes $2,000, the buyer has $2,000 less earning a return somewhere else. If you give the calculator an expected annual return, it compounds that cash difference monthly over the horizon and charges the growth to whichever option tied up more cash:

opportunity cost = extra cash down × ((1 + annual return ÷ 12)^months − 1)

It's optional because the honest version depends on what you'd truly do with the cash — and "I would definitely have invested it" is one of personal finance's most popular fictions.

A full worked example

The same example the calculator's built-in self-check verifies. A $40,000 car, 8% sales tax, compared over 36 months. (All numbers are illustrative inputs, not typical rates — yours come from your own quotes.)

With these particular numbers, leasing comes out about $4,186 ahead over 36 months (about $4,440 after charging the buy side ~$255 of opportunity cost on its extra $2,000 down at a 4% assumed return). Change the resale estimate, the residual, or the horizon and the verdict can flip — which is the whole point of doing the arithmetic instead of arguing.

What's not counted, and why

• Fees. Acquisition (~lease start), disposition (~lease end), doc, registration. Real, but they vary by state and lender; add yours to the matching side.
• Insurance and maintenance. Often similar between the options for the same car over the same window; where they differ (leases can require higher coverage), the difference is yours to add.
• Mileage and wear charges. The lease's sharpest hidden edge: overage commonly runs roughly $0.15–$0.30 per mile. If your driving is unpredictable, that risk belongs in your thinking even though no formula can price your odometer in advance.
• Residual as % of price vs. MSRP. Technically, residuals are set as a percentage of MSRP, while the cap cost uses your negotiated price. This calculator applies your residual percentage to the price you enter; if you negotiated far below MSRP, compute the residual dollars from MSRP yourself and back out the equivalent percentage.
• Multiple lease cycles vs. long ownership. This tool compares one lease term. The strongest case for buying is usually the years after the loan ends — rerun the comparison over a longer horizon to see it.

Educational estimate, not financial advice. Ready to run your own numbers? Back to the calculator.