Revenue is vanity; profit is sanity. A food truck can ring up $4,000 on a festival Saturday and still lose money once the commissary rent, ingredients, permits, and card fees are tallied. Profit -- what remains after costs -- is the only number that determines whether a business survives, and confusing it with revenue is the most common rookie mistake in small business.

The tool distills the whole period into one verdict: feed it revenue and costs, and the Result field declares profit or loss. Enter your total revenue for the period in the Variable A field and your total costs in Variable B -- a weekend's $4,000 in sales against $3,200 in expenses, say -- and the Result shows the bottom line. Positive means the period paid you; negative means you subsidized your customers.

The discipline is in what counts as a cost. Owners routinely forget their own labor, the home internet powering the Etsy shop, or the mileage to the supplier. List every expense honestly for a month, run the real totals through the tool, and you may discover the best-selling product is the least profitable one. Businesses rarely die from low sales; they die from sales that cost more than they bring in.

With the pre-filled defaults — Variable A 10, Variable B 5 — the Profit Calculator returns Result 50.0000. Change any input and those results update instantly, so you can see exactly how each number you enter moves the answer.