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First project: a public calculator with a clear answer

Scope a useful calculator before adding accounts or an AI service.

Last materially reviewed 2026-09-18

Quick answerA deterministic calculator is often a good first experiment because you can verify its answers independently.
What to know

Choose a calculation somebody needs

Start with a question that has a known formula and a useful interpretation, such as monthly software cost per active user. Define the units, assumptions and expected precision. Do not select a medical, investment or eligibility calculation as an easy beginner project. Those domains can require expertise and safeguards far beyond whether the arithmetic works. Choose something you can explain accurately without presenting a forecast as a fact.

What to know

Separate inputs from assumptions

List what the visitor supplies and what the formula assumes. A currency input needs a currency label; a time estimate needs a period. Show hypothetical defaults as examples, not observed customer data. Avoid collecting an email address just to display an answer. For a simple public tool, local calculations may be enough; an account, database or paid model call should solve a demonstrated need.

What to know

Write expected examples before generation

Calculate several cases independently: an ordinary value, zero where permitted, a missing field and an invalid denominator. Record the expected message as well as the number. Ask the builder to preserve the input while explaining an error. Check rounding at the final display rather than silently changing the underlying units. This is an original evaluation scenario, not a claim that Overskill produces correct calculators without testing.

What to know

Make the result useful and accessible

Explain what the number includes and excludes, then provide a relevant next decision. Test keyboard entry and enlarged text so the explanation remains usable outside your own screen. Keep the first version free of unrelated lead capture and unnecessary animation. If a static implementation meets the need, compare that simpler option before paying for an app-building platform. A small reliable tool is more valuable than an elaborate interface with unclear assumptions.

Continue when useful

Next: What does your app cost per useful user?

Use a simple planning calculation without pretending that projected users are observed traffic.

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Related guides

What does your app cost per useful user?Use a simple planning calculation without pretending that projected users are observed traffic.Make a generated app usable without a mouseCheck keyboard paths, readable text and clear form feedback before wider use.

Sources used for this page

These records support the facts and comparisons above. Merchant-controlled records are labelled so you can separate product claims from independent evidence.

  1. Overskill product page — Merchant documentation · overskill.com · Merchant-controlled · checked 2026-09-18
  2. MDN accessibility introduction — Reference · developer.mozilla.org · Publisher independence not verified · checked 2026-09-18