Assumptions

Pricing stays stable

The calculator assumes that prices remain stable for the period you specify. It does not model price increases, promotional expirations, or renegotiations. If you expect pricing to change, adjust the inputs to reflect your best estimate or run multiple scenarios.

For subscriptions, you can use either monthly or annual pricing. Annual pricing is normalized to a monthly equivalent so the totals are comparable. If the annual plan includes discounts, those savings are captured in the lower monthly equivalent.

We also assume the subscription price does not change midstream. If you expect price increases, model a higher average monthly price or run two comparisons: one for the current price and one for the price you expect after the increase.

Usage is consistent

We assume you use the product or service consistently throughout the usage months you enter. There is no partial usage or pausing built into the model. If you plan to pause, reduce the usage months accordingly so the calculation matches the months you actually pay or use the product.

This assumption matters because break-even depends on time. Extending or shrinking the usage period often changes the recommendation. If you are uncertain, test both a conservative and optimistic timeline.

Taxes, fees, and extras

Taxes, fees, shipping, and add-ons are not included automatically. Enter them directly in the price fields if they apply. This keeps the calculator flexible across regions and scenarios but requires you to include your real costs.

For example, if a subscription requires a mandatory support package, add that to the monthly or annual price. If a one-time purchase has installation fees, add those to the one-time price.

Discounts are applied evenly

If you enter a discount rate, the calculator applies it evenly across the subscription price or one-time purchase. It does not model expiring discounts or tiered pricing. If you have a short-term promo, you can model it by reducing the usage months or running separate scenarios.

Upfront discounts apply only to the one-time purchase. This is useful for coupons, rebates, or incentives that lower the initial cost.

No time value of money

The calculator does not discount future cash flows. It compares totals using simple arithmetic, not net present value. That keeps the model easy to understand, but it means the results are best used for practical decisions rather than formal financial analysis.

If you want to incorporate interest or financing costs, you can approximate them by increasing the monthly price or adjusting the one-time cost.

This is especially relevant for large purchases such as solar panels or equipment. Financing spreads costs over time and can blur the line between a subscription and a purchase, so make sure the inputs reflect the payment structure you will actually use.

Inflation and external factors

We do not model inflation or changes in the value you receive from the product over time. If you expect the value to decline or improve, that is a separate decision factor. The calculator focuses on cost rather than perceived value, which helps keep results consistent across scenarios.

Some services change their feature sets or add benefits over time. If those improvements matter to you, treat them as a qualitative factor after the numerical comparison.

Rounding and edge cases

We round totals to two decimal places for clarity. If the totals are extremely close, the recommendation may appear as a tie. In those cases, consider other factors like flexibility, features, or risk.

If you enter zero or negative values, the calculator treats them as zero to avoid misleading outputs. This keeps the model stable but means that invalid inputs will not produce meaningful results.

What this means for you

These assumptions are intentional. They keep the calculator focused on the trade-off most people face: how long to pay monthly versus paying upfront once. If your situation includes additional complexities, treat the calculator as a baseline and adjust the inputs to reflect your real costs.

If you want a deeper explanation of the math, visit the How It Works page. For questions about what is not covered, see the Disclaimer.