Your car's engine turns only about 30% of gasoline's energy into motion — the rest leaves as heat and noise. That 30% is its efficiency. Efficiency is useful output divided by total input, usually as a percent: a furnace delivering 80,000 BTU of heat from 100,000 BTU of gas runs at 80% AFUE. Nothing reaches 100% — thermodynamics guarantees some waste — and the gaps are big: LED bulbs convert about 90% of electricity to light versus 10% for old incandescents, which is why the same brightness costs a tenth of the power.

Americans meet efficiency ratings on yellow EnergyGuide labels: SEER for air conditioners, MPG for cars (itself an efficiency in disguise), and AFUE for furnaces. Businesses use the same ratio for processes — output per labor dollar. Because no machine is perfect, comparing two efficiencies is often a buying decision: the pricier model usually pays for itself in energy savings. Enter the useful output and the total input and the calculator returns the percentage, plus the wasted remainder.

With the pre-filled defaults — Variable A 10, Variable B 5 — the Efficiency Calculator returns Result 50.0000. Change any input and those results update instantly, so you can see exactly how each number you enter moves the answer.