How do you calculate resistance?

You need to use the formula E = IR + Ir

where: E is the e.m.f. of the power supply (the theoretical maximum voltage across the terminals when no current is flowing)

I is current

R is resistance of the circuit (load resistance)

and r is the internal resistance of the power supply.

Therefore, you can rearrange this formula to give r:

E = IR + Ir (Subtract IR)

E-IR=Ir (divide by I)

(E-IR)/I=r or r=(E-IR)/I