Communication Systems
Decibel & dBm Converter
Convert between linear ratios, dB, dBm, and watts, or cascade stage gains and losses to a running power level.
About this calculator
The decibel compares two power levels on a logarithmic scale: G = 10·log₁₀(P₂/P₁). Because the scale is logarithmic, gains and losses that would multiply in linear terms simply add in dB — which is why an entire receive chain can be summed on one line. dBm is the absolute cousin of the relative dB: power referenced to 1 mW, so 0 dBm is 1 mW, 30 dBm is 1 W, and −30 dBm is 1 µW. A level in dBm plus a gain in dB is again a level in dBm.
The classic trap is the factor of two: power ratios convert with 10·log₁₀, but amplitude ratios — voltage or current — convert with 20·log₁₀, because power is proportional to the square of amplitude in a fixed impedance. A ×10 voltage ratio is 20 dB; a ×10 power ratio is 10 dB. This converter keeps the two conventions as separate, labeled choices so the factor can never slip in silently.
The cascade mode chains up to three stage gains (enter losses as negative dB) onto an input level and shows the running level after each stage — the arithmetic core of every link budget and receiver line-up. For a full link with antenna gains, path loss, and margin against sensitivity, use the dedicated link budget calculator.
Assumptions
- dB values are power ratios (10·log₁₀); amplitude ratios use the separate 20·log₁₀ option.
- The 20·log₁₀ amplitude conversion assumes the two amplitudes appear across the same impedance.
- Cascade stages are linear and impedance-matched, so stage gains in dB add directly.
When to use this calculator
Appropriate for
- Converting between linear power/amplitude ratios and decibels, or between watts and dBm
- Summing a cascade of gains and losses in dB along a signal chain
- Teaching the 10·log (power) versus 20·log (amplitude) distinction
Not suitable for
- Mixing power and amplitude ratios in one calculation without tracking which convention each term uses
- Systems where source and load impedances differ, so a voltage ratio does not equal the power ratio
- Absolute level budgets without confirming the reference (dBm, dBW, dBμV) is consistent throughout
What this calculator does not cover
- The 20·log₁₀ amplitude form holds only between points at the same impedance — across an impedance transformation the power dB and voltage dB values differ.
- Cascade addition models ideal matched linear stages; it does not account for mismatch loss, VSWR interaction between stages, or gain compression at high drive levels.
- Noise is not tracked through the chain — cascaded noise figure requires the Friis noise formula, not plain gain addition.
- As with every calculator on this site: results are preliminary and educational, are not verified for any specific installation, and must be reviewed against the applicable code edition and stamped by a licensed Professional Engineer before real-world use.
Frequently asked questions
What is the difference between dB and dBm?
dB is relative — it compares two power levels, like the gain of an amplifier. dBm is absolute — power referenced to 1 mW, so 0 dBm = 1 mW and 30 dBm = 1 W. You add dB gains to a dBm level to get a new dBm level; adding dBm to dBm has no physical meaning.
When do I use 10·log₁₀ versus 20·log₁₀?
Use 10·log₁₀ for power ratios and 20·log₁₀ for amplitude (voltage or current) ratios. They agree because power is proportional to amplitude squared in a fixed impedance: a ×10 voltage ratio is a ×100 power ratio, and both come out as 20 dB.
Why do gains and losses simply add in dB?
Because logarithms turn multiplication into addition. Cascaded stages multiply their linear gains; in dB the same cascade is a sum, which is why a receiver line-up or a link budget can be totalled on one line.
References
- IEEE Std 100, The Authoritative Dictionary of IEEE Standards Terms — definitions of decibel and dBm
- Pozar, D. M., Microwave Engineering, 4th ed. — decibel notation and gain cascading
Related calculators
Link Budget Calculator
Received power and link margin from transmit power, antenna gains, feed losses, path loss, and receiver sensitivity.
Free-Space Path Loss (Friis) Calculator
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Thermal Noise & Noise Figure Calculator
kTB thermal noise power, noise power spectral density, noise temperature from noise figure, and the receiver noise floor.
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