Formula with three quantities
Many formulas have three quantities, where one is the other two multiplied together. If you know two of them, you can always find the third: cover the one you want in the triangle and see whether to multiply or divide.
The explanation is shown. Quick: pick a formula and enter two numbers. Advanced: your own names and units — and what the answer does when one of the numbers changes.
Your numbers
Choose a formula
The formulas are definitions from physics and mathematics — no table values.
More options: your own names and units
Write your own formula in the form A = B · C: give each quantity a name and a unit.
A
B
C
Answer
I (current)
3A
Formula rearranged
I = U / R
The formula
U = R · I
How to work it out
- 1U = R · I
- 2I = U / RCover I in the triangle: U is above R — so you divide.
- 3I = 12 V / 4 Ω = 3 AThe numbers must be in the units the formula uses. If you have mA or km, for example, convert first.
- 4Test: R · I = 4 · 3 = 12 = U ✓Put the answer into the original formula — both sides must give the same.
The formula
A = B · C
The quantity at the top of the triangle is the two bottom ones multiplied together.
B = A / C
One of the bottom ones: the top one divided by the other bottom one.
C = A / B
In depth: more numbers and “what if”
What if one of the numbers changes?
| Change | I (current) | The answer changes |
|---|---|---|
| U · 0.5 | 1.5 A | · 0.5 |
| U · 0.9 | 2.7 A | · 0.9 |
| U · 1.1 | 3.3 A | · 1.1 |
| U · 2 | 6 A | · 2 |
| R · 0.5 | 6 A | · 2 |
| R · 0.9 | 3.33 A | · 1.111 |
| R · 1.1 | 2.73 A | · 0.909 |
| R · 2 | 1.5 A | · 0.5 |
I follows U but goes the opposite way to R: twice as large R gives half.
More: Equations and functions
- EquationsSolve equations and inequalities step by step — and isolate a quantity in a formula.
- FunctionsQuadratic function, inverse proportionality and power function — with a graph.
- Linear functiony = ax + b as a graph and table — the line through two points or the line of best fit.
- Scale and ratiosFrom drawing to reality, area and volume to scale, the rule of three and dividing in a ratio.
- Two equations with two unknownsSolve a system of equations step by step and see the intersection on the graph.
- Growth and decayPercentage growth, the growth rate from two measurements, and doubling time.