# Build or buy calculator: when does your replacement cost less?

> Compare a SaaS bill with building your own replacement. Count development, migration, maintenance, and overlap to estimate the first cheaper month.

Author: [Flavio Copes](https://flaviocopes.com/about/) | Published: 2026-10-30 | Topics: [Business](https://flaviocopes.com/tags/business/) | Canonical: https://flaviocopes.com/build-or-buy-calculator/

A smaller hosting bill can make a homemade replacement look cheaper before you've counted the work needed to keep it running.

This calculator puts that work beside the subscription. Enter the time needed to build and migrate, the monthly maintenance, and the period when both systems stay available. You get cumulative costs and the first full month when building costs less.

It follows [Build vs buy software: when AI can replace a SaaS](https://flaviocopes.com/build-or-buy/). The guide explains how I would make the decision. This page lets you try the arithmetic with your own estimates.

## Include the work on both sides

The subscription side includes the monthly bill and the time you already spend managing that product. Leave those hours at zero if there is no relevant ongoing work.

The replacement side includes hosting and other services, maintenance hours, and the initial build and migration. Use the same hourly value for both sides.

That hourly value turns time into a comparable cost. It doesn't mean money leaves your bank account every time you work on the application. The calculator also shows cash bills separately, so you can see the distinction.

Include model API calls in the relevant service cost. A cheaper coding tool can reduce the cost of building, but it doesn't remove the time needed to inspect the result.

## What the example shows

The starting example uses a hypothetical €80 monthly subscription, €8 of replacement services, twelve initial hours, and one maintenance hour each month. Time is valued at €30 an hour.

Without an overlap period, the replacement costs €360 initially and €38 each month. After twelve months, that's €816 against €960 for the subscription, a €144 saving.

The default includes one month running both systems. That adds another €80, so the replacement's twelve-month total becomes €896. The estimated saving falls to €64.

At month ten, the subscription has cost €800 and the replacement €820. At month eleven, the totals are €880 and €858. The replacement first costs less at the end of month eleven.

These are invented inputs for exploring the calculation. Replace them with the work and bills your project would require.

## How the break-even number works

The comparison starts when the replacement is ready. All initial costs appear at month zero. The following rows add complete months of operation.

Let `B` be the monthly cost of keeping the subscription, including your time. Let `R` be the replacement's monthly running cost, also including time. `U` is the initial replacement cost, and `O` is the overlap in whole months.

After `M` months:

```text
Keep SaaS = B × M
Build = U + R × M + B × min(M, O)
```

During the overlap, you pay for both systems and do the estimated ongoing work for both. Afterward, the potential monthly saving is `B − R`.

When that saving is positive, the estimated equal-cost point is:

```text
Equal-cost month = (U + B × O) / (B − R)
```

The headline gives the first complete month with a positive saving. If both totals are equal at month ten, it says month eleven. Changing the comparison period changes the totals and chart, while the eventual crossover can remain beyond the displayed period.

If the replacement's recurring cost is equal to or higher than the subscription's, there is no recurring saving to repay the initial work. The calculator shows that state rather than inventing a payback date.

## Change the uncertain inputs

Maintenance is easy to underestimate. Try twice the hours you first entered and see whether the replacement still costs less within a useful period.

Also try a longer overlap. You may need time to compare output, finish an export, or discover an unusual workflow before cancelling the original service.

The model holds monthly costs constant. Inputs accept up to two decimal places. It rounds each time-cost component to cents, with half cents rounded up, and leaves out financing, inflation, taxes, and changes in usage. A delayed build can also postpone the date when you start saving. The calculator starts at readiness, so estimate that calendar date separately.

A numerical saving doesn't establish that the replacement handles your important cases. Export your data, check the recovery path, and try the ordinary work before giving up the system you depend on.

The form runs in your browser. Valid inputs are placed in the page URL for sharing. Changing the currency changes its label; it doesn't convert the estimates.