I'd like to increase the muffin recipe to make 18 muffins instead of 12. I know this is simple math but what is exactly the formula? Multiply each ingredient by 1.5?
generally, yes. Easy for liquid and dry measures. A bit harder for egg. If I'm multiplying and I have a choice, I make enough to use whole eggs. Thus, if 1 doz requires 1 egg, I'll make double batch using 2 eggs and then freeze extras. If you can't/don't want to do that, use 1 egg white for half an egg. It will give you slightly more than half the volume, and help with the rising/expansion of the dough.
Ok...they are in the oven. I'll let you know how they turn out. I have made this recipe a million times but once I tried to increase it and it was a total flop. I know it's just basic math, but I need them for a neighborhood meeting this afternoon.
Yes. Your math is correct. When you encounter a bizarre quantity, simply round up to the next most logical unit. In the matter of eggs, if you encounter a fraction over 1, round up to 2, and so forth.
Sometimes as well, with recipes written volumetrically, leavening doesn't scale well. Yet another reason to use metric measurements....the scaling is much more accurate.