Determining DC gain of single supply op-amp (no AC component)
I have a formula of 1+(R2/R1), but that doesn't seem to be valid here.


It's perfectly valid; \$\dfrac{V_O}{V_I} = 1+\dfrac{R_2}{R_1}\$

Think about how an op-amp works when negative feedback is applied; the output, via R2 and R1 sets the inverting input to equal the voltage on the non-inverting input. In doing so the output must be higher because of the potential dividing effect of R1 and R2.

The op-amp is like a servo system. You set a target voltage on Vin and the output finds a level to make -Vin almost exactly match +Vin. Ignoring DC offset errors (a mV or so) the closeness that the output pushes the two inputs together is determined by open-loop gain which, for an op-amp is generally about 1,000,000 (this can be more or it can be less depending on the op-amp chosen).

If R2 is 5 times bigger than R1 then Vout is 6 times bigger than -Vin. If +Vin is 1 volt then the output is 6 volts in order to maintain the two inputs at the same voltage (with an error because of small dc offsets and the op-amp gain not being infinite).