Gross-Up Calculator: Promise a Net, Pay the Right Gross

The problem this kills

Someone in leadership promises an employee "an extra $1,000 in their pocket." Now it's your problem. Because that $1,000 is the net — the take-home after taxes — and payroll runs on gross. So you open a spreadsheet, guess at the supplemental tax rate, divide, add FICA, remember the state, redo it, and end up either shorting the employee or over-paying and explaining the variance later.

It gets worse with relocation checks, taxable reimbursements, and gift cards that have to be "grossed up" so the person isn't taxed on a benefit they were told was free. The math isn't hard, but it's fiddly, it's easy to use last year's rate, and there's no record of which assumptions you used when finance asks three weeks from now.

You don't need to be a developer to fix this. You can build a small, careful tool that does the backward math the same way every time, shows its work, and won't let a number reach payroll until a human signs off on it.

What you'll build

A private web app for your payroll team where you:

• Enter the desired net (the take-home you promised) and pick the employee and the pay item type (spot bonus, taxable reimbursement, relocation, gift card grossed-up, and so on).
• Pick the tax method — supplemental flat rate or aggregate — and the tool pulls the right federal, FICA, and state rates for that employee's work state.
• See the full breakdown instantly: the gross to pay, each tax line that comes out, and the resulting net (which should match your promise to the penny), with rounding applied consistently and the assumptions spelled out.
• Send it to your payroll manager, who reviews the gross and the rates used and approves — only then does the grossed-up line get added to a batch or exported.
• Export the approved line in your payroll provider's exact column layout, plus a one-line plain-English summary you can paste into a ticket or email.

What's inside the Implementation Plan

The plan opens by interviewing you about your business — it does not assume a generic payroll setup. Before it builds anything, the AI asks how you run pay today, which provider columns you export to, which tax method your company uses for supplemental pay, your federal/FICA/state rates and wage caps, how you name and identify employees, your typical and peak bonus volumes, and the messy edge cases (employees who've hit the Social Security wage cap, multi-state workers, zero-state-tax states, "net zero" gift cards). Then it reads a short tailored spec back to you and waits for your thumbs-up. The tool you get fits your payroll, not a template.

From there the plan walks you, copy-paste prompt by copy-paste prompt, through standing up the database, the gross-up math (done the careful backward way, with consistent rounding you can defend), the entry screen, the approval gate, and the export — all the way to a working tool you can use this week.

The governance it includes (this is the point)

This isn't a loose spreadsheet macro. Every build includes:

• Login so only your team can open the tool.
• Row-level security so each company only ever sees its own employees and calculations.
• A complete audit trail — who computed which gross-up, with which rates, who approved it, and when.
• A hard human-in-the-loop approval gate: the AI computes and drafts, your payroll manager reviews the gross and the exact assumptions, and only an approved calculation can be exported or added to a run.
• Duplicate guards so the same bonus event for the same employee can't be grossed up and paid twice.

Who it's for

Payroll and comp admins who get handed a take-home number and have to reverse-engineer the gross — the people who promise "an extra $500 in their pocket" and then have to make payroll actually deliver it, defensibly and on time.

You've got this. Paste the first prompt and let the interview tailor it to your payroll.