Ohm's law relates voltage, current, and resistance. The electrical power equation relates voltage, current, and power. Combining them lets two positive known values determine the other two for one ideal resistive DC element. The algebra is compact, but the measurement boundary matters as much as the symbols.
Start with V = I × R
Voltage across the element equals current through it multiplied by resistance. Rearranging gives I=V/R and R=V/I. Use values referring to the same two terminals and the same branch; mixing a supply voltage with one component's current can produce a numerically tidy but physically unrelated answer.
Add P = V × I
For the stated resistive DC relationship, electrical power is voltage multiplied by current. Substituting V=IR gives P=I²R; substituting I=V/R gives P=V²/R. Those are algebraic consequences, not separate physical models.
Six known pairs cover the formula wheel
V+I, V+R, V+P, I+R, and I+P use only rational multiplication and division. R+P requires I=√(P/R) and V=√(PR). A square root can be irrational, so a trustworthy calculator must distinguish exact rational results from displayed approximations.
A formula is not a circuit design
The four quantities do not determine topology, frequency response, tolerance, temperature rise, transient current, insulation, wiring, or a component rating. Treat the result as arithmetic for an ideal operating point and verify every real-world constraint separately.