The 13 month rule
There are 13 months in a rental year. That's how the rental house charges you, and it's how you should charge for the machines you already own.
A rental month is 4 weeks, not a calendar month. 52 weeks divided by 4 is 13. Keep a machine out for a full year and the rental house bills you 13 monthly rates, so your own monthly rate has to be built on 13 periods too. Spread your annual ownership cost across 12 and every machine you own under-recovers by about 8.3%.
Calendar months aren't rental months
| Job says | Weeks on site | Rental periods to price |
|---|---|---|
| 1 month | 4.333333333333333 | 1.08 |
| 2 months | 8.666666666666666 | 2.17 |
| 3 months | 13 | 3.25 |
| 6 months | 26 | 6.5 |
| 9 months | 39 | 9.75 |
| 12 months | 52 | 13 |
Read the last row. A machine on site for a calendar year is 13 periods, not 12. Every duration in between carries the same 1.083 multiplier, and it's the reason a job that finished on schedule can still come in under on equipment recovery.
The cost basis is your floor, not your price
Your equipment cost basis is what a machine costs you to own and operate: ownership duration, replacement cost, general maintenance, insurance and registration, major repairs, divided by real annual working days. That number is the floor. Everything above it is either covering the cost of the work happening around the machine or contributing to profit.
Quote at the floor and you've priced a job that runs to plan and no longer. Construction jobs run longer. The 13 period convention is one of the places the buffer lives, because it prices the way the machine is billed and the way a general contractor is already comparing you against a rental quote.
Three moves, in order
The template is Equipment cost basis worksheet, one of the 6 that ship with CONTROL. Already know your cost basis and want to price a rate against it? SPM The Construction CFO keeps the equipment cost calculator.
Step 02: Equipment cost basis
A true internal rate per machine covering ownership, maintenance, fuel, and transport, charged to the projects that used it.
What this costs when it is missing
What owners ask
Why do equipment rentals charge 13 months in a year?
Because a rental month is 4 weeks, not a calendar month. There are 52 weeks in a year, and 52 divided by 4 is 13. So a rental house that keeps a machine out for a full year collects 13 monthly rates, not 12. That's the industry convention, and it's the convention your own iron should be priced on.
What does the rule change about my rate?
It changes the divisor. If you spread a machine's annual ownership cost across 12 periods and then bill it on a 28 day cycle, you recover less than the machine costs you, every year, on every job. Divide the annual cost by 13 periods and the rate you quote matches the cycle you bill on.
How do I apply it to a job duration?
Convert calendar time to rental periods before you price it. A 6 calendar month job is 26 weeks, which is 6.5 rental periods. Price 6 and you're short by a quarter of a period on a job that ran to plan.
Does this apply to weekly and daily rates too?
The daily rate is the one job costing runs on, and it comes from annual working days, not from this rule. The 13 period rule bites the moment you quote by the week or the month, because those are the units a rental house prices in and the units a general contractor compares you against.
Is the cost basis the price, or the floor?
The floor. Your equipment cost basis tells you what the machine costs you to own and run. Everything above that line is either recovering the cost of the job around it or contributing to profit. Quoting at the floor means a project that runs long costs you money, and construction projects run long.
Where does this sit in the system?
Step 02, equipment cost basis, which is chapter 2 of CONTROL: The Construction Financial Operating System. A true internal rate per machine covering ownership, maintenance, fuel, and transport, charged to the projects that used it.
Josh Luebker ran commercial construction projects before he ran the numbers behind them, on data centres, military bases, hospitals, and high-rises. He wrote CONTROL: The Construction Financial Operating System, which is an eight-step framework for commercial subcontractors, and it publishes on October 1, 2026. He is a master electrician who moved into the office and kept the field vocabulary.
Sulphur Prairie Operations LLC maintains the benchmark reference behind these figures. Every edition is reconciled against the newest CFMA survey release before it publishes here. Where a figure changes between editions, the older edition keeps its year in the title so a citation to it stays correct.