Calculator
Example data table
| Power (W) | dBW | dBm | Notes |
|---|---|---|---|
| 0.001 | -30 | 0 | 1 mW equals 0 dBm. |
| 1 | 0 | 30 | 1 W equals 0 dBW. |
| 10 | 10 | 40 | Tenfold power adds 10 dB. |
Formula used
- dB = 10 × log10(P / Pref)
- dBW uses Pref = 1 W
- dBm uses Pref = 1 mW = 0.001 W
- Custom reference uses your chosen Pref
How to use this calculator
- Select the conversion mode you need.
- Enter one value, or paste multiple values.
- Pick your power unit, then set a reference.
- Choose precision and press Submit to calculate.
- Use CSV or PDF buttons to export results.
Watts to decibels guide for practical work
1) Why decibels are used with power
Decibels compress huge ranges of power into small, readable numbers. A transmitter might move from 0.001 W to 1000 W, which is a million‑to‑one ratio. In dB, that same change is 60 dB, so spreadsheets and field notes stay tidy. It also simplifies error budgets and comparisons across test runs quickly today.
2) Absolute levels depend on the reference
Power in dB becomes “absolute” only when you choose a reference. dBW uses 1 W as the reference, so 1 W = 0 dBW, 10 W = 10 dBW, and 100 W = 20 dBW. dBm uses 1 mW, so 1 mW = 0 dBm and 1 W = 30 dBm.
3) Ratios vs. levels: dB, dBW, and dBm
dB by itself is a ratio: 10·log10(P2/P1). dBW and dBm are levels: 10·log10(P/Pref). This calculator supports both styles, so you can convert a single measured power to a level, or compare two powers as a gain or loss.
4) Key “data points” worth memorizing
Three rules save time: +3 dB is about double the power, −3 dB is about half, and +10 dB is ten times the power. Example: 2 W is about 3.01 dBW, 5 W is about 6.99 dBW, and 50 W is about 16.99 dBW.
5) Handling units correctly
Watts, milliwatts, microwatts, and kilowatts are just scaling factors. Converting the unit first prevents mistakes. For instance, 250 mW is 0.25 W, which equals −6.02 dBW and +23.98 dBm. When you paste multiple values, each value is normalized before conversion.
6) Precision, rounding, and reporting
Engineering work often reports 1–2 decimals for dB (because measurements vary), but lab work may need 3–6 decimals. The precision selector lets you match your reporting needs. For very small differences, extra decimals help you see real changes instead of rounding noise.
7) Common uses in RF, audio, and optics
RF link budgets add gains and losses in dB; audio power amps are compared using dB changes; optical power meters often read in dBm for convenience. Converting watts to dB helps you combine components (cables, filters, amplifiers) without multiplying long decimals.
8) Avoiding typical conversion mistakes
Most errors come from mixing voltage and power formulas, or using the wrong reference. Use 10·log10 for power (watts), not 20·log10 (voltage/current). Also confirm whether your target is dBW (1 W) or dBm (1 mW) before interpreting the result.
FAQs
1) What does 0 dBW mean?
0 dBW means the power equals the reference of 1 watt. Positive dBW is above 1 W, and negative dBW is below 1 W.
2) What does 0 dBm mean?
0 dBm means the power equals 1 milliwatt. It is commonly used in RF and fiber optics because many practical power levels sit near 0–30 dBm.
3) Why is 1 watt equal to 30 dBm?
dBm uses 1 mW as reference. Since 1 W = 1000 mW, 10·log10(1000) = 30 dB, so 1 W = 30 dBm.
4) When should I use dB (ratio) instead of dBW/dBm?
Use dB when you’re comparing two powers as gain or loss, such as amplifier gain, cable attenuation, or filter insertion loss. Use dBW/dBm for absolute power levels.
5) Can decibels be negative?
Yes. A negative level means the power is below the chosen reference. For example, 0.1 W is −10 dBW, and 0.01 mW is −20 dBm.
6) Why does +3 dB mean “about double”?
Because 10·log10(2) ≈ 3.0103 dB. It is close enough for quick engineering estimates, especially when measurement uncertainty is larger than a few hundredths of a dB.
7) Why do voltage conversions use 20·log10 instead of 10·log10?
Voltage and current relate to power through a square relationship (P ∝ V² or I²). The logarithm of a square becomes a factor of 2, so the coefficient becomes 20 instead of 10.