# dBm Calculator

dBm calculator converts watts, milliwatts, Vrms, dBV, and dB to and from dBm. Includes 50 ohm and custom impedance. dBm to watt table reference for RF and radio work.

## What this calculates

dBm is the ratio of a power level to 1 milliwatt, expressed logarithmically in decibels. RF engineers, radio hams, and audio techs all use dBm because logarithmic units compress the huge dynamic range between microwatt receivers (-60 dBm) and kilowatt transmitters (+60 dBm) into a readable scale. This dBm calculator handles every common conversion: watt to dBm, dBm to watt, milliwatt to dBm, dBm to Vrms, dBV to dBm, and applying dB gain or loss offsets to a dBm reading.

## Inputs

- **Conversion** — options: Watt to dBm, dBm to watt, Milliwatt (mW) to dBm, dBm to Vrms, dBV to dBm, dB offset applied to dBm — Pick the conversion direction. Radio, RF, and audio engineering all use the same equations.
- **Power (watts)** (W) — min 0 — Used for Watt to dBm. 1 W = 30 dBm, 2 W = 33 dBm, 5 W = 37 dBm, 10 W = 40 dBm.
- **Power (milliwatts)** (mW) — min 0 — Used for milliwatt to dBm. 1 mW = 0 dBm by definition.
- **Value (dBm or dB)** (dBm) — Used for dBm to watt, dBm to Vrms, dB to dBm, and dBV to dBm.
- **dB offset** (dB) — Positive adds gain, negative adds loss. Used in dB to dBm mode.
- **Impedance** (Ω) — min 1 — System impedance for dBm to Vrms. 50 ohm is standard RF, 75 ohm is video, 600 ohm is legacy audio.

## Outputs

- **Result (dBm)** (dBm) — Power level in dBm relative to 1 milliwatt.
- **Result (watts)** (W) — Linear power equivalent of the dBm value.
- **Result (milliwatts)** (mW) — Power in mW.
- **RMS Voltage** (V) — Vrms = sqrt(P x Z). Populated for dBm to Vrms mode.
- **Formula Used** — formatted as text — The specific dBm formula applied for this conversion.

## Details

The dBm formula

The definition of dBm is logarithmic:

  - dBm = 10 x log10(P_mW) where P_mW is power in milliwatts.

  - Equivalently: dBm = 10 x log10(P_W x 1000) = 10 x log10(P_W) + 30.

  - Solved for power: P_W = 10^(dBm / 10) / 1000.

That means 1 mW = 0 dBm, 1 W = 30 dBm, 1 kW = 60 dBm, and -30 dBm = 1 microwatt. Every +10 dBm means 10x the power; every +3 dBm means 2x the power.

dBm to watt table

  
    dBmWattsCommon use
  
  
    -90 dBm1 pWFaint radio signal at receiver input
    -60 dBm1 nWTypical weak cellular signal
    -30 dBm1 microwattStrong indoor WiFi at client
    0 dBm1 mWBluetooth Class 2 (reference)
    20 dBm100 mWLow-power WiFi AP
    30 dBm1 WFCC Part 15 2.4 GHz limit, HT DMR radio
    33 dBm2 W2 watt to dBm - handheld HT limit
    37 dBm5 W5 watts to dBm - GMRS and FRS portable
    40 dBm10 W10 watt to dBm - mobile VHF/UHF radio
    47 dBm50 WVehicle mobile radio
    60 dBm1 kWAmateur radio HF linear amplifier limit
  

Watt to dBm conversion examples

A watt to dBm calculator runs dBm = 10 x log10(W x 1000). Quick reference: 0.5 W = 26.99 dBm, 1 W = 30 dBm, 2 W = 33.01 dBm, 3 W = 34.77 dBm, 5 W = 36.99 dBm, 10 W = 40 dBm, 25 W = 43.98 dBm, 100 W = 50 dBm. Notice that doubling the watts adds about 3 dB: 1 W -> 2 W = +3.01 dB, 5 W -> 10 W = +3.01 dB.

dBm to Vrms conversion

dBm is a power unit; Vrms is a voltage unit. To go between them you need the system impedance. The equation is Vrms = sqrt(P_W x Z) = sqrt(10^(dBm/10) / 1000 x Z). In 50 ohm (standard RF), 0 dBm = 0.2236 Vrms = 223.6 mVrms. In 75 ohm video, 0 dBm = 0.2739 Vrms. This calculator accepts any impedance via the impedance field.

dBV to dBm conversion

dBV is voltage relative to 1 Vrms; dBm is power relative to 1 mW. Converting dBV to dBm requires impedance because voltage to power depends on Z: dBm = dBV + 10 x log10(1000 / Z). In 50 ohm that offset is +13.01 dB (so 0 dBV = 13.01 dBm). In 600 ohm (legacy audio) the offset is +2.218 dB.

dB to dBm (cascaded gain and loss)

dB is a dimensionless ratio; dBm is an absolute power level. You can add dB to dBm because log-power + log-ratio = log-power. A +20 dB antenna amplifier on a -30 dBm signal gives -30 + 20 = -10 dBm. A -3 dB splitter on 40 dBm gives 37 dBm out each leg. This db to dbm calculator mode applies any dB offset to a dBm starting value.

## Frequently Asked Questions

**Q: How do I convert watts to dBm?**

A: dBm = 10 x log10(P_mW) = 10 x log10(P_W x 1000). Examples: 1 W = 30 dBm, 2 watt to dBm = 33.01 dBm, 5 watts to dBm = 36.99 dBm, 10 watt to dBm = 40 dBm. Doubling the wattage adds 3 dB. This watt to dBm calculator handles any power value.

**Q: What is the dBm to watt formula?**

A: Power in watts = 10^(dBm / 10) / 1000. For a 10 watts to dBm example run in reverse: 40 dBm = 10^(40/10) / 1000 = 10,000 / 1000 = 10 W. The dBm to watts table shows common values at a glance.

**Q: How do I convert dBm to Vrms?**

A: Vrms = sqrt(P_W x Z), where Z is impedance. For 0 dBm (1 mW) in 50 ohm: Vrms = sqrt(0.001 x 50) = 0.2236 V = 223.6 mV. The dBm to vrms calculator mode supports any impedance, including 75 ohm video and 600 ohm legacy audio.

**Q: Is 2 watt to dBm just 3 dB higher than 1 watt?**

A: Yes. 2 watt to dBm = 10 x log10(2000) = 33.01 dBm, exactly 3.01 dB higher than 1 W (30 dBm). Every doubling of power adds 3 dB. 4 W = 36 dBm, 8 W = 39 dBm, 16 W = 42 dBm. That 3 dB rule is the fastest mental shortcut in RF.

**Q: How do I add a dB gain or loss to a dBm signal?**

A: dBm_out = dBm_in + dB_offset. A +20 dB amplifier on a -10 dBm signal gives +10 dBm. A -6 dB cable loss on 30 dBm drops to 24 dBm. This db to dBm calculator mode handles any cascaded gain or loss chain.

**Q: What is the difference between dBV and dBm?**

A: dBV is voltage relative to 1 Vrms. dBm is power relative to 1 mW. Converting dBV to dBm requires impedance: dBm = dBV + 10 x log10(1000 / Z). In 50 ohm RF, the offset is +13.01 dB (so 0 dBV = 13 dBm). This dbv to dbm conversion is built into the calculator.

---

Source: https://vastcalc.com/calculators/physics/dbm
Category: Physics
Last updated: 2026-04-08