Or, not and
Almost every line on a service list carries two intervals — so many miles or so many months — and the schedule runs on whichever arrives first. Which one that is depends entirely on how much you drive, and it is not the same answer for every line on the same list.
Converting is one step. Divide annual mileage by twelve to get miles per month, multiply by the month interval, and compare against the mileage interval:
Effective interval = the smaller of (mileage interval) and (months × annual miles ÷ 12)
At 12,000 miles a year that is 1,000 miles a month. A 5,000 mile or 6 month oil interval becomes 5,000 or 6,000, so miles govern. A 15,000 mile or 12 month filter interval becomes 15,000 or 12,000, so months govern — and that is the line people get wrong, because it is the one where the mileage figure is the larger and more memorable number.
What that does to a budget
The default list on this page — oil, rotation, filters, brake fluid, coolant — over a 60,000 mile window at 12,000 miles a year comes to 30 services and $1,474, which is $294.80 a year and 2.457 cents a mile.
| Item | Intervals | Effective | Times in 60,000 mi | Total |
|---|---|---|---|---|
| Engine oil and filter | 5,000 mi or 6 mo | 5,000 (miles) | 12 | $624.00 |
| Tire rotation | 6,000 mi or 6 mo | 6,000 (together) | 10 | $250.00 |
| Cabin and engine air filters | 15,000 mi or 12 mo | 12,000 (months) | 5 | $240.00 |
| Brake fluid exchange | 30,000 mi or 36 mo | 30,000 (miles) | 2 | $220.00 |
| Coolant exchange | 60,000 mi or 60 mo | 60,000 (together) | 1 | $140.00 |
Look at the filter line. Budget it on the 15,000 mile interval alone and it comes due four times in the window instead of five, and the line reads $192 instead of $240. That is $48 missing from a five year budget on one item, from reading the wrong column. Repeat that across a longer list and the shortfall stops being trivia.
The effect gets stronger the less you drive. At 6,000 miles a year the same filter interval becomes 6,000 effective miles rather than 15,000, so it comes due two and a half times as often per mile driven, and the same is true of every time-triggered item on the list. Low-mileage vehicles are not cheap to maintain per mile; they are expensive per mile, and the arithmetic above is why.
How the counting works
Given a last-done odometer reading and an effective interval, the next occurrence is simply the sum of the two. Occurrences inside a window are then a floor and an addition:
Times due = floor((odometer + horizon − next due) ÷ effective interval) + 1, and zero if the next one falls past the end of the window
For the oil line: next due at 76,200 + 5,000 = 81,200, window ending at 78,500 + 60,000 = 138,500, so floor(57,300 ÷ 5,000) + 1 = 11 + 1 = 12. The plus one is the occurrence at the start of the run, and forgetting it is the classic fencepost error in this kind of count.
An item already past due still counts inside the window, because it is still work that has to happen. It shows with the mileage it went past rather than a distance until.
What the schedule is inferring
Time since the last service is inferred from annual mileage rather than taken from dates, which keeps the form short and is exactly right for a vehicle driven at a steady rate. It is wrong for a vehicle that sat for a season, because the calendar kept running while the odometer did not, and the month triggers on such a vehicle arrive earlier than this arithmetic shows. If that describes your car, treat the month-governed lines as due sooner than the figure here.
Everything else on the page is your own data. The intervals are yours, from your documentation; the costs are what those jobs actually cost you; the last-done figures are your records. Nothing here is a recommendation about what your vehicle needs or how often, and no interval on this page originated on this page.
Before the car goes in the air
A jack lifts a vehicle and does not hold one. Jacks sink into warm asphalt, they slide off a pinch weld that was already folded over, and people are killed every year by cars that came off them. Rated stands, hard level ground, the wheels still down chocked, and nothing goes underneath until the vehicle has been shoved hard at the bumper and did not move. These pages are arithmetic and contain no lifting procedure on purpose, because a paragraph on a website is not how anybody should learn that.
Related pages
For what an oil change itself costs per change and per mile, including comparing two intervals against each other, see the oil and fluid cost calculator; if the engine uses oil between changes, the oil consumption calculator turns that into a rate and a top-up budget. Individual big-ticket lines have their own pages: brake job planning from measured pad and rotor thickness, coolant mix and drain volume, and tread wear for when the tires land. The car maintenance schedule covers what usually sits on a list like this, and car ownership cost puts the yearly figure above beside depreciation, fuel and insurance. Machines measured in hours rather than miles have their own versions: engine hours and service intervals and small engine service intervals.
Questions people ask
Which interval do I follow, the mileage one or the months one?
Whichever arrives first, which is what the word between them means. That is not a general rule this page is inventing — it is how service schedules are written, and it is why they carry two numbers instead of one. What the calculator adds is the conversion, because comparing 15,000 miles against 12 months requires knowing how far you drive. At 12,000 miles a year the month figure wins on that line; at 20,000 miles a year the mileage figure wins on the same line and the same car. What your vehicle actually requires is in your own service documentation, and there are usually separate schedules for severe-duty conditions that this page has no knowledge of.
Why does driving less make maintenance cost more per mile?
Because the time-triggered items keep coming due while the mileage-triggered ones do not. A 12 month item at 12,000 miles a year lands once every 12,000 miles; at 6,000 miles a year the same item lands once every 6,000 miles, so per mile driven it costs twice as much. Meanwhile the fixed costs of owning the vehicle at all are spread over half the distance too. The arithmetic on this page makes it visible: change the annual mileage figure and watch which lines switch from being governed by miles to being governed by months. It is one of the reasons per-mile cost figures for low-mileage vehicles look surprising.
What if something on my list has no mileage interval, only months?
Leave the mileage field blank and the calculator uses the month interval alone, converted at your annual mileage. It works the other way too — a line with only a mileage interval and no month figure runs on mileage alone. Items with neither are skipped rather than guessed at. That matters because some things on a real list genuinely are calendar-only, and forcing a mileage figure onto them to make a form happy produces a schedule that is wrong in a way you will not notice later.
Does this tell me what my car needs?
No, and it is worth being clear about it. Every interval on this page is one you typed in, and the defaults are placeholders that exist so the arithmetic has something to work on. Service requirements are set by the manufacturer, they differ by engine, by transmission, by model year and by whether the vehicle qualifies for a severe-duty schedule, and they change over time as manufacturers revise them. The document that governs is the one that came with your vehicle or the current manufacturer schedule for it. This calculator is a spreadsheet with the fencepost arithmetic already done.
Should I combine services to save a trip?
The next-twelve-months list on this page exists to make that visible, because when four items all fall due within a few thousand miles of each other it is usually cheaper and less annoying to do them together, especially when they share access — anything requiring the wheels off, for instance. What the arithmetic will not tell you is whether pulling an item forward to bundle it is a good idea, since doing something early costs the unused portion of its interval and doing it late is a different decision entirely. Both are judgement calls about your vehicle. The numbers just show you where the clusters are.