The short version: No tomato paste on hand? Use 3 tablespoons of tomato sauce for every 1 tablespoon of paste the recipe calls for. That's it. Toss it in, keep cooking, you're golden. 3:1, Sauce:paste.
I'm not guessing here — I made enchilada gravy with this exact swap today. Recipe wanted 1 tablespoon of paste, I only had sauce, 3 tablespoons of sauce nailed it. Perfect.
For those of you like me who have to know the science to fully buy in — here it is:
Tomato paste is just tomato sauce with the water cooked out of it. It's roughly three times as concentrated — same tomato solids, sugars, acids, and all that deep umami flavor, just packed into a third of the volume. So when you swap in 3 tablespoons of sauce for 1 of paste, you're delivering the exact same amount of actual tomato — you've only changed how much water it's riding in. And in anything that simmers (gravy, chili, sauce), that extra water just cooks right off. You end up in the same place the paste would've taken you. It's not a magic ratio — you're just reversing what the factory did when they boiled the sauce down into paste.
Going the other way? (You have paste, need sauce)
Flip the math: tomato paste thinned with water becomes sauce. Use the empty paste can as your measuring cup — fill it with water and stir it in until it looks right. A 6 oz can of paste plus about a cup of water, simmered 10 minutes, gives you roughly an 8 oz can's worth of sauce. Start thicker; you can always add water, but you can't take it back out.
One honest warning
This swap is fantastic for cooked dishes — chili, enchilada gravy, spaghetti, stews, anything that simmers. It's not meant for raw uses (skip it on bruschetta), and thinned paste can curdle in creamy sauces like vodka or alfredo. For those, hold out for the real thing.