01 · Total costCompare total cost, not only the price today
A cheaper upfront option can become more expensive once recurring fees, energy use, maintenance, financing or replacement frequency are included. A useful comparison uses the same time horizon for both options and includes costs that materially differ between them.
For products you may later sell, resale value can reduce the effective ownership cost. For subscriptions and services, annualizing a monthly price helps reveal the scale of a recurring commitment.
02 · Cost per useAsk what each real use costs you
Cost per use is useful when comparing something you own with something you rent, subscribe to or pay for as needed. Divide the relevant ownership or subscription cost by realistic expected uses—not by an optimistic best case.
If your expected usage is uncertain, calculate a low, normal and high-use scenario. A result that flips between those scenarios is a sign that usage is the key driver.
03 · Time valueKeep time visible without pretending every hour is money
Time can matter in commuting, cooking, travel, DIY and repetitive work. DecisionNumbers lets time be explicit because ignoring it can hide a real trade-off. But not every saved hour should automatically be priced at your wage.
If assigning money to time feels inappropriate, set the value-of-time input to zero and compare cash separately. Then treat the time difference as a second result rather than forcing both dimensions into one number.
04 · Break-evenFind the point where paying more starts to make sense
Break-even answers a simple question: how long, how many uses or how much distance must pass before the higher upfront option catches up? This is often more useful than declaring one option universally better.
A break-even point beyond the expected useful life or your planned ownership period is a warning that the theoretical saving may never be realized in your situation.
05 · SensitivityChange the inputs most likely to change the answer
Every estimate depends on assumptions. Instead of trusting a single result, change uncertain inputs one at a time. Fuel price, resale value, usage, repair life, subscription frequency and time horizon often have outsized effects.
If a modest change reverses the result, the decision is close. That is useful information: it means non-financial factors such as convenience, reliability or flexibility may deserve more weight.
06 · False precisionRound the conclusion when the inputs are rough
A calculator can display exact numbers even when the assumptions are estimates. Treat a result as an estimate range when your inputs are uncertain. A difference of a few dollars should not be presented as decisive when the underlying prices could vary by much more.
Decision support is strongest when it makes uncertainty visible instead of hiding it behind extra decimal places.
07 · Local contextUse your local prices, currency and units
Defaults are examples, not claims about your city or country. Taxes, energy prices, transport costs, salaries and service fees vary substantially. Replace examples with your own current numbers whenever the difference matters.
The platform architecture separates country, currency, language and units so future localized versions can reuse the same underlying decision without duplicating the core logic.
08 · BoundariesKnow when a calculator should not make the decision
Some choices depend on safety, medical facts, legal duties, personal values or regulated professional advice. A cost comparison can organize part of such a decision, but it should not replace qualified advice or a factual assessment that the calculator cannot perform.
DecisionNumbers therefore focuses on transparent, low-risk comparisons and keeps higher-risk professional judgments outside the scope of automated recommendations.