The recipe says 9-inch round. You own a 13×9.
One multiplier, from the ratio of pan areas — the arithmetic behind every published pan chart, for any two pans instead of the eight the chart printed. Everything stays in your browser.
The recipe's pan
Your pan
The day
The numbers here are yours to check. This page does arithmetic on what you type — it cannot see your recipe, your equipment or your batch.
For anything you will eat or sell as food, double-check it against your own process and your local rules. This page does maths, not food safety.
Want it to remember?
In MadeKeeper the scaled recipe becomes the batch: ingredients consume from your pantry at the multiplied quantities, the batch knows its cost, and the bake prices itself. See for cottage food makers or poke around the live demo. $19/mo for the whole back office, kept for as long as you stay. The shop stays free for cost a piece and one fair — no product cap. The tax packet, Etsy and Square, and more fairs are the $19.
The pan math every recipe assumes you know
'Can I make this in a 13×9' has one honest answer, and it is arithmetic. The math below is the same math the calculator runs — pan areas and their ratio — worked in the open so you can check it against the chart on your fridge.
How does pan conversion actually work?
By area, at equal depth. A 9" round is about 64 square inches; a 13×9 is 117; the ratio — about 1.8 — is your multiplier for every ingredient. That is the entire secret behind every pan-conversion chart ever printed: they computed the areas so you would not have to. This page computes them for any two pans, including the odd ones no chart bothers with.
Do I need to change the bake time?
When the scale factor is big, yes — same temperature, earlier first check. A batter spread across more area runs shallower and bakes faster; crowded into less area it runs deeper and slower. Inside roughly two-thirds to one-and-a-half times, the original time is a good starting point. The calculator flags which case you are in.
What about pans of different depths?
That is a baking judgment, not arithmetic, and this page is honest about the line. Area math assumes the batter depth stays roughly the same. Converting a shallow sheet to a deep round changes how the crumb sets, whether the center domes, and how long the middle takes — a chart cannot carry that judgment and neither can we. Scale by area, then bake like the baker you are.
Can I scale any recipe this way?
Most everyday bakes, yes — butter cakes, brownies, bars, quick breads. The edges: very large scale factors strain leavening ratios (doubling works better than quintupling), delicate foam cakes (angel food, chiffon) depend on pan geometry itself, and yeasted breads follow dough weight rather than pan area. For the ordinary case — the recipe pan you do not own — the multiplier is exactly what you need.