Resistors Formulas
Resistor formulas cover how resistance combines in series and parallel and how to size current-limiting resistors for components such as LEDs. These calculators add, combine, and reverse-solve resistor networks so you can pick standard values quickly. They are essential for biasing, current limiting, and building precise resistance values from common parts.
Resistors Formula Calculators
- Series Resistance Rtotal = R1 + R2 + ... + Rn Resistor Values 0.47 kΩ, 1.03 kΩ, 2.21 kΩ → Rtotal = 3.71 kΩ
- Parallel Resistance 1/Rtotal = 1/R1 + 1/R2 + ... + 1/Rn Resistor Values 0.47 kΩ, 1.03 kΩ, 2.21 kΩ → 1/Rtotal = 281.6
- LED Resistor R = (Vs − Vled) / Iled Source Voltage 3.3 V, LED Forward Voltage 1.55 V, LED Current 1 mA → Resistance = 1.75 kΩ
- Resistor Power Derating Pmax = Prated × (Tmax−Tamb)/(Tmax−Trated) Rated Power 22 mW, Max Temperature 2.5 °C, Ambient Temperature 5.5 °C, Rated Temperature 11.8 °C → Derated Power = 7.097 mW
- Resistive Attenuator Atten = 20 × log10(R2 / (R1 + R2)) Series Resistor 1 Ω, Shunt Resistor 2.2 Ω → Attenuation = -3.255 dB
- Resistor Tolerance Range R_max = R × (1 + tol) Nominal Resistance 1 Ω, Tolerance 0.5 % → R_max = 1.005
- Resistor Thermal Noise (Johnson–Nyquist) Vn = √(4 × k × T × R × BW) Temperature 2.5 °C, Resistance 1 Ω, Bandwidth 5 Hz → Noise Voltage = 0.2759 nV
- Potentiometer Output Vout = Vin × Rpos / Rtotal Input Voltage 0.726 V, Wiper Position 1 Ω, Total Resistance 2.2 Ω → Output Voltage = 0.33 V
- Delta-to-Wye (Δ→Y) Transform R_Y = (Ra × Rb) / (Ra + Rb + Rc) Delta Ra 1 Ω, Delta Rb 2.2 Ω, Delta Rc 4.7 Ω → R_Y = 0.2785
- Nearest E24 Resistor Value Round to nearest standard E24 value Target Resistance 1 Ω → Round to nearest standard E24 value = 1
Worked result for every Resistors formula
| Formula | Expression | Worked result |
|---|---|---|
| Series Resistance | Rtotal = R1 + R2 + ... + Rn | Resistor Values 0.47 kΩ, 1.03 kΩ, 2.21 kΩ → Rtotal = 3.71 kΩ |
| Parallel Resistance | 1/Rtotal = 1/R1 + 1/R2 + ... + 1/Rn | Resistor Values 0.47 kΩ, 1.03 kΩ, 2.21 kΩ → 1/Rtotal = 281.6 |
| LED Resistor | R = (Vs − Vled) / Iled | Source Voltage 3.3 V, LED Forward Voltage 1.55 V, LED Current 1 mA → Resistance = 1.75 kΩ |
| Resistor Power Derating | Pmax = Prated × (Tmax−Tamb)/(Tmax−Trated) | Rated Power 22 mW, Max Temperature 2.5 °C, Ambient Temperature 5.5 °C, Rated Temperature 11.8 °C → Derated Power = 7.097 mW |
| Resistive Attenuator | Atten = 20 × log10(R2 / (R1 + R2)) | Series Resistor 1 Ω, Shunt Resistor 2.2 Ω → Attenuation = -3.255 dB |
| Resistor Tolerance Range | R_max = R × (1 + tol) | Nominal Resistance 1 Ω, Tolerance 0.5 % → R_max = 1.005 |
| Resistor Thermal Noise (Johnson–Nyquist) | Vn = √(4 × k × T × R × BW) | Temperature 2.5 °C, Resistance 1 Ω, Bandwidth 5 Hz → Noise Voltage = 0.2759 nV |
| Potentiometer Output | Vout = Vin × Rpos / Rtotal | Input Voltage 0.726 V, Wiper Position 1 Ω, Total Resistance 2.2 Ω → Output Voltage = 0.33 V |
| Delta-to-Wye (Δ→Y) Transform | R_Y = (Ra × Rb) / (Ra + Rb + Rc) | Delta Ra 1 Ω, Delta Rb 2.2 Ω, Delta Rc 4.7 Ω → R_Y = 0.2785 |
| Nearest E24 Resistor Value | Round to nearest standard E24 value | Target Resistance 1 Ω → Round to nearest standard E24 value = 1 |
Variables used across Resistors formulas
- Rtotal — Total equivalent resistance (Ω)
- R1, R2, ... Rn — Individual resistor values (Ω)
- Vsource — Supply voltage (V)
- VLED — LED forward voltage drop (V)
- ILED — Desired LED forward current (A)
- R — Required series resistance (Ω)
- Pmax_derated — Maximum allowable power at operating temperature (W)
- Prated — Rated power at reference temperature (W)
- Tmax — Maximum component temperature (°C)
- Tambient — Actual ambient temperature (°C)
- Trated — Temperature at which full power is rated (°C)
- Atten — Attenuation in decibels (dB, negative value)
- R1 — Series resistor (Ω)
- R2 — Shunt resistor to ground (Ω)
- tol — Tolerance as a ratio (e.g., 0.05 for 5%)
- Vn — RMS noise voltage (V)
- T — Temperature (°C, converted to Kelvin internally)
- BW — Measurement bandwidth (Hz)
- Vout — Output voltage at the wiper (V)
- Vin — Voltage across the full potentiometer (V)
- Rpos — Resistance from ground to wiper (Ω)
- Ra — Delta resistor between nodes 1-2 (Ω)
- Rb — Delta resistor between nodes 2-3 (Ω)
- Rc — Delta resistor between nodes 1-3 (Ω)
Questions people ask about Resistors
- What is the formula for tolerance for resistance? — Resistor Tolerance Range
Related Formula Categories
Browse the full formulas index for all 32 categories.