Convert volts to mA: enter voltage and resistance or power to get current in milliamps, find volts from mA, or turn battery watt-hours into mAh.
| Watts to mA | mA to Watts |
|---|---|
| 0.1 W = 20 mA | 10 mA = 0.05 W |
| 0.25 W = 50 mA | 30 mA = 0.15 W |
| 0.5 W = 100 mA | 75 mA = 0.375 W |
| 1 W = 200 mA | 150 mA = 0.75 W |
| 2 W = 400 mA | 250 mA = 1.25 W |
| 2.5 W = 500 mA | 350 mA = 1.75 W |
| 3 W = 600 mA | 700 mA = 3.5 W |
| 5 W = 1000 mA | 1200 mA = 6 W |
| 7.5 W = 1500 mA | 1800 mA = 9 W |
| 10 W = 2000 mA | 2200 mA = 11 W |
| 12 W = 2400 mA | 2600 mA = 13 W |
| 15 W = 3000 mA | 5000 mA = 25 W |
| mA = (W / V) x 1000 | W = (V x mA) / 1000 | |
| Watts to mA | mA to Watts |
|---|---|
| 1 W = 83.3 mA | 50 mA = 0.6 W |
| 3 W = 250 mA | 150 mA = 1.8 W |
| 5 W = 416.7 mA | 300 mA = 3.6 W |
| 10 W = 833.3 mA | 500 mA = 6 W |
| 15 W = 1250 mA | 800 mA = 9.6 W |
| 18 W = 1500 mA | 1000 mA = 12 W |
| 25 W = 2083.3 mA | 2000 mA = 24 W |
| 35 W = 2916.7 mA | 2500 mA = 30 W |
| 50 W = 4166.7 mA | 4000 mA = 48 W |
| 60 W = 5000 mA | 6000 mA = 72 W |
| 75 W = 6250 mA | 7500 mA = 90 W |
| 100 W = 8333.3 mA | 10000 mA = 120 W |
| mA = (W / V) x 1000 | W = (V x mA) / 1000 | |
| Watts to mA | mA to Watts |
|---|---|
| 0.5 W = 20.8 mA | 10 mA = 0.24 W |
| 1 W = 41.7 mA | 50 mA = 1.2 W |
| 2 W = 83.3 mA | 100 mA = 2.4 W |
| 5 W = 208.3 mA | 250 mA = 6 W |
| 10 W = 416.7 mA | 500 mA = 12 W |
| 24 W = 1000 mA | 1000 mA = 24 W |
| 48 W = 2000 mA | 2000 mA = 48 W |
| 72 W = 3000 mA | 3000 mA = 72 W |
| 120 W = 5000 mA | 5000 mA = 120 W |
| mA = (W / V) x 1000 | W = (V x mA) / 1000 | |
| Voltage | 100 Ω | 220 Ω | 470 Ω | 1 kΩ | 10 kΩ |
|---|---|---|---|---|---|
| 3.3 V | 33 mA | 15 mA | 7 mA | 3.3 mA | 0.33 mA |
| 5 V | 50 mA | 22.7 mA | 10.6 mA | 5 mA | 0.5 mA |
| 9 V | 90 mA | 40.9 mA | 19.1 mA | 9 mA | 0.9 mA |
| 12 V | 120 mA | 54.5 mA | 25.5 mA | 12 mA | 1.2 mA |
| 24 V | 240 mA | 109.1 mA | 51.1 mA | 24 mA | 2.4 mA |
| 48 V | 480 mA | 218.2 mA | 102.1 mA | 48 mA | 4.8 mA |
| 120 V | 1200 mA | 545.5 mA | 255.3 mA | 120 mA | 12 mA |
| Resistance values are standard E24 series resistors. | |||||
| Component | Voltage | Current |
|---|---|---|
| Standard LED | 2-3.3 V | 10-20 mA |
| High-power LED | 3-3.6 V | 350-700 mA |
| Arduino Uno | 5 V | 40-50 mA |
| ESP32 (TX) | 3.3 V | 160-260 mA |
| 4-20 mA loop | 12-24 V | 4-20 mA |
| DC motor (hobby) | 3-6 V | 100-300 mA |
| Servo (SG90) | 5 V | 100-550 mA |
| 5 V relay coil | 5 V | 70-80 mA |
| USB 2.0 configured standard port | 5 V | 500 mA |
| USB 3.0 configured standard port | 5 V | 900 mA |
| USB-C PD phone | 5-20 V | 1500-3000 mA |
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Volts to mA Formulas
There are two primary formulas for converting volts to milliamps, depending on whether you know the circuit’s power consumption or its resistance.
Method 1: Using Power (Watts)
Where P is power in watts and V is voltage in volts. This method is practical when working with device power ratings, which are commonly printed on power supplies and appliance labels. A 60 W light bulb on a 120 V circuit draws (60 / 120) x 1000 = 500 mA.
Method 2: Using Resistance (Ohm’s Law)
Where V is voltage in volts and R is resistance in ohms. This is derived directly from Ohm’s Law (I = V/R) and is the standard approach in circuit design. When resistance is expressed in kilohms, the formula simplifies to mA = V / R(kilohms), eliminating the multiplication step entirely. 5 V across a 1-kilohm resistor yields exactly 5 mA.
Where Volts-to-mA Conversions Are Used
LED Circuit Design
LEDs require appropriate current control, which may use a resistor or a constant-current driver. For a resistor-limited indicator circuit, the volts-to-mA calculation is useful. A standard indicator LED has a forward voltage of roughly 2 V and a rated current of 20 mA. To drive it from a 5 V supply, you subtract the forward voltage (5 – 2 = 3 V across the resistor) and apply Ohm’s Law: R = 3 V / 0.020 A = 150 ohms. Use the actual LED current/forward-voltage specification and check resistor power dissipation; a generic example is not a component rating. High-power LEDs used in automotive and architectural lighting operate at 350 mA to 1 A or more, making precise current calculation critical for thermal management.
4-20 mA Industrial Control Loops
The 4-20 mA current loop is the dominant signaling standard in process control. Sensors for temperature, pressure, level, and flow transmit their readings as a current between 4 mA (representing 0% of scale) and 20 mA (representing 100% of scale). The loop is powered at 12 V to 24 V DC. A key advantage of current signaling over voltage signaling is noise immunity: series-loop current is maintained within the transmitter’s specified operating limits. Available supply voltage must cover transmitter requirements and all wire/receiver drops; noise and installation conditions can still affect performance. The 4 mA lower bound also serves as a fault indicator. Below4 mA can represent underrange or a configured fault; interpret the device’s saturation and alarm settings. For example, Emerson’s Rosemount manual distinguishes NAMUR low saturation at3.8 mA from fail-low at3.6 mA or below. A live-zero signal helps distinguish zero process value from an open loop within a specified diagnostic scheme.
USB Power Delivery and Charging
Permitted USB current depends on the port, device and configured or negotiated connection. Conventional configured high-power standard downstream ports permit500 mA for USB2.0 and900 mA for USB3.0 peripherals on USB3.0 ports; these are not universal limits for every USB charging connection. See USB-IF compliance policy. The USB Battery Charging spec allows up to 1,500 mA from a dedicated charging port. USB Power Delivery (USB-PD), common on USB-C connectors, negotiates voltage and current levels dynamically, supporting up to5 A at48 V (240 W) under USB-PD3.1 EPR with a compatible negotiated device, charger and cable rating. See USB-IF’s Power Delivery overview. A phone negotiating fast charge at 9 V and 2,000 mA receives 18 W, while a laptop charger operating at 20 V and 3,000 mA delivers 60 W.
Battery Life Estimation
Battery capacity is rated in milliamp-hours (mAh), making mA the natural unit for calculating runtime. A device drawing 50 mA from a 2,000 mAh battery lasts approximately 2,000 / 50 = 40 hours under ideal conditions. In practice, battery voltage drops as it discharges, which changes the current draw of voltage-regulated circuits. Knowing the exact mA draw at the operating voltage is essential for accurate runtime predictions in portable devices, remote sensors, and IoT nodes.
DC vs. AC: How the Conversion Differs
The formulas above apply directly to DC (direct current) circuits, where voltage and current are constant. In AC circuits, voltage and current vary with time and can be distorted or out of phase. True power factor accounts for the relationship between real and apparent input power. For purely resistive AC loads (heaters, incandescent bulbs), the DC formulas work if you use the RMS (root-mean-square) voltage, which is the value reported by standard multimeters. For a sine wave,120 V RMS corresponds to a peak voltage of about170 V, but the RMS value is what matters for power and current calculations.
For AC loads with significant inductance or capacitance (motors, transformers, fluorescent ballasts), a power factor term is needed: mA = (P / (V x PF)) x 1000, where PF is the power factor ranging from 0 to 1. A load consuming100 W of real electrical input power at120 V RMS with a power factor of0.8 draws (100 / (120 x 0.8)) x 1000 = 1,042 mA, not the 833 mA you would calculate ignoring power factor.
Current and Electrical-Shock Risk
Electrical-shock effects depend on current magnitude, duration, current path, waveform and contact conditions. Human-body impedance is variable and nonlinear; it is not a fixed resistor. IEC 60479-1 describes different time-current zones and pathways for AC and DC. A single milliamp value cannot establish a universal effect or a safe contact limit.
| Factor | Why it matters |
|---|---|
| Current magnitude | Possible effects range from sensation and muscle reaction to burns, cardiac or respiratory injury; no universal current-only threshold determines the outcome. |
| Contact duration | The time current flows changes the risk; current magnitude and time must be considered together. |
| Current path | The route through the body changes which organs are exposed. |
| Waveform and frequency | AC and DC have different effect zones; frequency and other waveform characteristics matter. |
| Skin and contact conditions | Moisture, skin damage, pressure and contact area affect body impedance. |
| Protective devices | Devices address particular faults and do not make all contact conditions safe. |
| Qualitative reference, not safe-contact limits. Sources: IEC 60479-1 and NIOSH Electrical Safety. | |
120 V can cause fatal shock, including in dry conditions. These calculations cannot determine whether contact is safe. A GFCI detects a small imbalance between outgoing and returning current, typically about5 mA; that operating principle is not derived from a universal DC body-current threshold. GFCIs do not protect against every shock hazard, including some simultaneous hot/neutral contacts. See OSHA’s GFCI guidance.
