Finance

What finance apps are worth building?

Start with a question people already ask themselves and that a spreadsheet answers badly. Good candidates cluster around three jobs: finding charges that leave an account without a purpose, checking whether a bill or price is right, and turning an irregular income into a per-day or per-hour figure.

The lowest-data version is almost always the one to build first. A tool that works from a typed number or an uploaded file is easier to trust and to explain, and it carries far less risk than one that connects straight to a bank.

Who uses money tools, and where do you find them?

The people most likely to use a money tool are those who already feel the cost. Freelancers who price their own time, households that renew subscriptions every year without noticing, and people who receive bills they do not understand are typical users. Expect people to use a tool once to get an answer and return only when the question comes back, so the question has to recur.

Reach them at the moment of the question. A subscription audit sits naturally next to the complaints people make about subscriptions. A cost calculator belongs in the freelance communities where people talk about rates. Test your headline with the exact sentence someone would type into a search box.

• Interview five people who have the problem and ask them to show you the last time it happened. Build the question around their words.
• Publish the calculator or checker free, with no sign-up, and count how many people reach the final answer.
• Treat a willingness to pay as real only when the payment buys a saved hour or a saved charge, not a promise.

How do you judge a finance idea?

A finance idea is good when its output is one number or one decision, and the user can check that number against a document they already trust.

• A single answer: “Three charges renewed this month that you did not mention last month” is one answer. A dashboard of spending categories is not.
• Checkable output: the user can compare the result with the statement or bill in front of them.
• Minimal data: the question needs the smallest possible set of inputs. Ask for the bill, not the bank login.
• A stated limit: the tool says what it cannot see, such as cash payments or accounts it was not given.
• A reason to return: the question recurs monthly, quarterly or at each renewal date.

What does a first finance tool look like?

Hypothetical example: a freelancer wants to know what a day without client work costs them in tools and unpaid admin. The first version asks four questions: day rate, monthly tool spend, unpaid admin hours each week, and paid weeks per year. It returns one figure for a non-billable day, with the formula shown underneath.

Build a single-page calculator. Inputs: day rate, monthly tool costs, unpaid admin hours each week, and paid weeks per year. Output: the cost of one non-billable working day, with the formula displayed underneath. No sign-up, no storage of inputs, and no tax calculations.

Test it by posting the calculator in one freelance community and watching what happens. If almost nobody reaches the answer, the question is not felt strongly enough. If people finish it and ask how to save their numbers, you have a case for a recurring product, which is a separate build with its own rules.

Which rules and risks apply to finance apps?

• Rules differ by country. Giving advice, lending, handling payments and holding other people’s money can each be regulated, and the rules change by country. Nothing on this page is legal or financial advice. Check the rules where you will operate before you recommend anything.
• Bank connections. Connecting to a bank account is the largest step in data sensitivity. Some regions have formal open-banking rules that only authorised providers may follow, so a direct connection is not something to bolt on casually.
• Recommendations that look like advice. A tool that tells someone which product to buy or which loan to take can be treated as regulated advice in some places. Keep the output to facts the user can verify.
• Wrong figures. A bill checker that is wrong once will be remembered. Show where every number came from, and say plainly when the tool is unsure.
• Data kept too long. Store the least you can. Uploaded statements should be deleted once the answer is shown, unless the user chooses to keep them.

Where should a finance builder start?

Three finance ideas are on the board at the time of writing, and each matches a job above. Subscription graveyard is the recurring-charge audit and a good test of the single-answer rule. Photograph any bill and see if you’re overpaying is the bill checker, which needs the checkable-output rule and the wrong-figures warning. What a freelance day actually costs you is the calculator from the worked example.

1. Pick one recurring money question you answered by hand this month.
2. Reduce it to one input and one output, then check the result against a real document.
3. If the question is not covered by the finance ideas already posted, describe your input, output and limits in a new post.
4. Read how to validate an app idea and how to price a micro-SaaS before you think about charging anyone.