Resistor Combination Calculator
Combine 1–3 resistors in series or parallel. Series: R=R1+R2+... Parallel: 1/R=1/R1+1/R2+.... 100Ω+200Ω series=300Ω, parallel≈66.7Ω.
Input Data
Results
At a glance:Resistors in series add: R=R₁+R₂+… . Resistors in parallel follow the reciprocal rule: 1/R=1/R₁+1/R₂+… (equivalently R=R₁R₂/(R₁+R₂) for two). Properties: (1) series raises the total resistance; (2) parallel lowers it below the smallest value; (3) equal parallel resistors halve the value (two) or divide by n. Applications: (1) making non-standard resistance values; (2) LED current limiting; (3) sensor voltage dividers; (4) gain-setting in op-amp circuits; (5) load sharing.
Formula
Series: R = R₁+R₂+R₃+…
Parallel: 1/R = 1/R₁+1/R₂+1/R₃+…
Two in parallel: R = R₁R₂/(R₁+R₂)
$$R_{s} = \sum R_i, \quad \frac{1}{R_{p}} = \sum \frac{1}{R_i}$$How to Use
- Pick series or parallel mode.
- Enter up to three resistor values (R₃ optional).
- The equivalent resistance is shown instantly.
Case Studies
LED current-limiting resistor
An LED drops ~2 V and needs 20 mA from a 5 V supply. Series resistor R=(5−2)/0.02=150 Ω.
Use 150 Ω or the nearest standard 160 Ω; power rating ≥0.06 W, so 1/4 W is safe.
Two 150 Ω in series give 300 Ω and halve the current — useful if a single value is unavailable.
Content review: Calculatorism Science Team. Results are for reference only; please refer to the relevant authorities for the official figures.