Handle money amounts correctly in code

Prompt that reviews how money amounts are stored and rounded and sets rules with test cases. Paste it into an AI tool of your choice, which then reviews how money amounts are handled.

Topic: Apps & prototypes
Type: Prompt
Level: Intermediate
Time: about 10 min

Results depend on the tool, the model and what you enter.

Result preview

Example: written by our editors, not real AI output.

Finding: cart.js, line 42 → price stored as float Rule: store amounts as whole cents, round only in calculate_total() Test: €100.00 in 3 instalments → 33.34 + 33.33 + 33.33 = 100.00 Question for the tax adviser: round tax per line or on the total?

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ApplicationWhere are amounts calculated?

StackWhat do you work with?

ProblemWhat have you noticed?

Your prompt

Review how the following application handles money amounts.
Application: [Application]
Stack: [Stack]
Problem: [Problem]
Approach: 1. Find every place where amounts are stored or calculated as floating-point numbers and propose whole cents or a decimal type. 2. Decide whether rounding happens per line or on the total, and state exactly where it happens. 3. Define one rounding rule and keep it in a single place in the code. 4. Check discounts, partial amounts and instalments so the parts add up exactly to the total. 5. Store the currency with every amount and keep display format separate from the stored value. 6. List test cases with amounts where rounding tips over.
Output: list of findings, rules for storing and rounding, test cases. Mark questions about tax or accounting rules as questions for a professional.

Take particular care to never treat amounts as floating-point numbers and define every rounding in exactly one place.

Before you answer, check: Are all instructions met? If key information is missing or a placeholder in square brackets is still in the text, ask instead of guessing. Mark assumptions as assumptions.

Review how the following application handles money amounts.
Application: [Application]
Stack: [Stack]
Problem: [Problem]
Approach: 1. Find every place where amounts are stored or calculated as floating-point numbers and propose whole cents or a decimal type. 2. Decide whether rounding happens per line or on the total, and state exactly where it happens. 3. Define one rounding rule and keep it in a single place in the code. 4. Check discounts, partial amounts and instalments so the parts add up exactly to the total. 5. Store the currency with every amount and keep display format separate from the stored value. 6. List test cases with amounts where rounding tips over.
Output: list of findings, rules for storing and rounding, test cases. Mark questions about tax or accounting rules as questions for a professional.

Take particular care to never treat amounts as floating-point numbers and define every rounding in exactly one place.

Before you answer, check: Are all instructions met? If key information is missing or a placeholder in square brackets is still in the text, ask instead of guessing. Mark assumptions as assumptions.

This is an instruction for an AI, not the result itself.

Where this prompt works

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How to use it

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Copy the finished prompt into an AI tool of your choice.
Split 100.00 into three and add the parts back up. If the result is not exactly 100.00, a rule for the remainder is missing.

Why does this work?

Floating-point numbers cannot represent many decimal amounts exactly, and every place rounds differently. A fixed data type, one rounding rule in one place and tests for edge cases prevent differences that are painful to track down later.

Tips

General technical help, not tax advice. If you are unsure how taxes must be rounded, check with your tax adviser.

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Handle money amounts correctly in code

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