Rental Income ROI Calculator

Project Deep Dive

This is a property investment worksheet built for an accounting firm's landlord clients, designed to turn a page of rental income and expense figures into a single, unambiguous profitability number. Before the rebuild, the sheet already handled the most common source of error in a DIY rental analysis: every line let the landlord state whether the number they typed was a monthly or a yearly amount, rather than assuming one convention for the whole page.

What the spreadsheet did

The workbook was a single calculations sheet with eight labeled input rows: rental income, followed by seven expense categories - property tax, property insurance, mortgage payments, utilities, repairs and maintenance, condo or management fees, and other expenses. Every row paired a dollar amount with a Yearly/Monthly selector, and a companion formula annualized that row on the spot with a simple conditional - multiplying by 12 only when the selector read Monthly - so a landlord could mix a yearly tax bill with a monthly mortgage payment without converting anything by hand. A summary cell then subtracted the total of the seven annualized expense rows from the annualized income row to produce one net income figure, with a documented rule that it should display in green when positive and red when negative. A side note flagged that, for Canadian tax purposes, only the interest portion of a mortgage payment is deductible, not the principal, so landlords needed to know which figure to enter if they wanted a tax-accurate result.

How we converted it

We rebuilt the eight input rows as a single form, each amount field paired with its own Yearly/Monthly dropdown exactly as the workbook laid them out, and ported the sheet's per-row annualizing formula into the same conditional logic in JavaScript, run inside one change handler bound to the whole form so every keystroke or dropdown switch recalculates instantly instead of waiting on a spreadsheet reopen. The net-income formula - income minus the sum of the seven annualized expenses - carries over line for line, and the result still follows the workbook's own color convention: green text for a profitable year, red for a loss. The one genuinely new piece is visual: a live pie chart, rebuilt on every recalculation, that breaks the same eight annualized figures down by category so a landlord can see at a glance which expense line is eating the most into their rental income - something the static spreadsheet had no way to show.

What changed for the client

For the firm's landlord clients, the payoff is a page that behaves exactly like the worksheet's own logic - same eight line items, same per-row monthly/yearly choice, same net-income formula - but recalculates instantly instead of requiring a spreadsheet reopen, and now shows the expense breakdown as a chart rather than a column of numbers. The color-coded result carries over the workbook's own visual convention, so anyone who understood the original sheet reads the web version the same way. The firm has since put the calculator on its own website as a tool prospective landlord clients can use directly.