One wrong number on a rushed estimate can turn a good job into a bad deal. If you're still pricing on a napkin, here's how I'd turn your rules into a calculator you can check and reuse.
How do I build a job cost calculator with ChatGPT?
Give ChatGPT your labor costs, material costs and pricing rules. Ask it to build a spreadsheet that separates inputs from formulas and shows how each total is calculated. Check the formulas against completed jobs before using it for a new quote.
A job calculator applies your pricing rules consistently. You still have to decide which costs belong in those rules and whether the final price makes sense.
What numbers should I give it?
- Labor: expected hours and your cost per hour. Label labor cost separately from the hourly rate you charge a customer.
- Materials: quantities, unit costs and any waste allowance you actually use.
- Other costs: applicable overhead, subcontractors, equipment, delivery and other job expenses. State how you allocate them.
- Pricing: whether you add a markup to cost or work toward a margin on the selling price. State how discounts, taxes and rounding should work where applicable.
Use anonymized examples and only business information you're allowed to share in your AI workspace. The calculator does not need customer names, payment details or confidential contracts to test its formulas.
What prompt can I use?
Replace the bracketed details with your own rules:
“Build a job-pricing spreadsheet using these rules: [labor calculation], [materials], [overhead and other costs], and [markup or target margin]. Ask about missing inputs before building. Separate inputs from formulas, label units, and show how every total is calculated. Flag missing or invalid values. Include a test sheet so I can compare three completed jobs with independently checked calculations. Do not invent rates or silently change my pricing rules.”
What is the difference between markup and margin?
Markup is the amount added to cost, expressed as a percentage of cost. Margin is the difference between the selling price and cost, expressed as a percentage of the selling price. The same percentage produces different prices.
Illustrative example, not a client result: eight hours at a labor cost of $40 an hour equals $320. Add $180 in materials and direct cost is $500.
- A 25% markup: $500 × 1.25 = $625. The $125 difference is 20% of the selling price.
- A 25% margin: $500 ÷ 0.75 = $666.67, rounded to the nearest cent.
Those figures leave out any costs not included in the $500. They are not a promise of net profit. Your calculator must include the costs and pricing rules your business actually uses.
How do I check the calculator before using it?
- Start with three completed jobs. Recalculate the expected totals independently, then compare every line and formula.
- Do not make the calculator match an old quote if that quote was wrong. Separate the formula check from a review of what the job actually cost.
- Test missing inputs, zero hours, decimal quantities and your markup or margin rule. Check discounts, taxes and rounding if your calculator includes them.
- Fix discrepancies and rerun the tests. Review each new quote before sending it.
Three successful examples are a starting point, not proof that every future quote will be right. For a broader review habit, see how to challenge AI output before you trust it.
What does this not solve?
A calculator will not fix missing costs or a pricing rule that loses money. It also cannot know how long your crew will take or what a customer will accept unless you supply reliable inputs. Your rates, your assumptions and the quote you send remain your decisions.
What should you do today?
Write down how you price a job, build the first version and check it against three completed jobs. If you're rebuilding the same estimate every time, talk with me about making that work repeatable.
Need better records of what you spent? Read how to match receipts to credit-card charges with ChatGPT.
Who is Jeff Fargo?
I'm Jeff Fargo, founder of Fargo Factor in Henderson, Nevada. I help business owners streamline and scale with AI and get found in AI search. Fargo Walks is where I share practical ways to put AI to work, one problem at a time.
Sources
- Fargo Walks, episode 118: Bad Math Robs Your Pay. Build a ChatGPT Calculator. Published November 1, 2026. This article expands on the clip with a sample prompt, a hypothetical calculation and checks to run before using a calculator. These additions are practical guidance, not a reported client result.
Transcript
You're still doing napkin math? Really? Every job. Same numbers. Labor. Materials. Markup. That's it. The only thing is, if you're doing a piece of paper or a napkin, you get one number wrong, you might be working for free.
Open up ChatGPT. Put everything in there that you use to figure out a job. Then have it build you a custom calculator. Yes, it can do that now. Have it do the calculator. Put your last three jobs in there. Check it. If it gets anything wrong, tell it what to do differently. Fix it.
How about that for your time back? I'm Jeff Fargo. I can help you streamline and scale with AI. Go forth.