Make an estimate calculator for a service business
A calculator can reduce repeated typing, but it can repeat a bad formula just as quickly. Start with a pricing rule the owner has already approved. Use made-up examples until the result is checked.
1. Write down the rule
For an illustrative example only, suppose a service charges a $40 call-out fee plus $30 per hour. Two hours would produce $100 before any other approved charges. These numbers are teaching examples, not pricing advice.
List exactly what the calculator includes and excludes. Decide how partial hours work, whether discounts are allowed, and who can change rates. Do not guess tax treatment or accounting rules.
2. Make inputs and rounding explicit
Use labeled fields for quantity, rate, and any approved extras. Reject impossible values and show what needs correcting. A blank field should not silently become an accepted zero.
For builders, use an appropriate decimal-money approach, such as integer minor units with deliberate rounding. Avoid uncontrolled floating-point calculations. Define when rounding occurs and test boundary values.
3. Show a review before export
Display the inputs, the calculation, the total, the date, and a clear “estimate” label. Let the person correct a value without starting over. Record which rate version was used so a later rate change does not rewrite an old estimate.
If saving locally, say so. Give the user a complete export or printable result with no claim that it has been sent to a customer. Sending is a separate action that needs confirmation of success.
4. Test known answers
Check zero, one unit, a partial quantity, a large quantity, and an invalid entry. Compare each result with an independently checked calculation. Test export and reopening on the device used for quotes.
Try the calculator offline if that is part of the promise. Keep a manual calculation method available until the owner has accepted the test results. Have the relevant professional review any regulated or accounting-specific rules.
Your choice stays on this device. It is not sent to the site owner.