How this instrument works
Electrical power delivered into any resistive load follows P = V² ÷ R, one of the standard forms of Ohm's law's power relationship. Applied to headphones, V is the source's output voltage and R is the headphone's impedance in ohms — a spec-sheet number describing how much the headphone resists current at a given frequency. Because headphones need far less power than speakers, the result is conventionally reported in milliwatts rather than watts, which is what the ×1000 in the formula converts to.
This is why voltage and impedance both matter for how a headphone-source pairing actually performs. A smartphone's headphone output or a laptop's built-in DAC typically supplies a limited voltage — plenty of power for common low-impedance consumer headphones and earbuds (16 to 32 Ω is typical), but that same limited voltage into a high-impedance studio or professional headphone (250 to 600 Ω is common) yields far less power, which is exactly why those headphones are usually paired with a dedicated headphone amplifier capable of higher output voltage.
Power delivered isn't the same thing as perceived loudness, though. A headphone's sensitivity rating — decibels of sound pressure level per milliwatt (dB SPL/mW) — varies substantially between models, so two headphones fed identical power can sound noticeably different in volume. This calculator answers the electrical question of how much power reaches the driver; how loud that power actually sounds depends on the specific headphone's own sensitivity spec as well.
- Enter Output voltage (V) — the source's rated or measured RMS output voltage into the load.
- Enter Headphone impedance (Ω) — the impedance figure from the headphone's spec sheet.
- Read Power delivered (mW) — the electrical power reaching the headphones at that voltage and impedance.
- To compare two sources, hold impedance fixed and change only voltage; to compare two headphones, hold voltage fixed and change only impedance.
- This is electrical power only — check the headphone's sensitivity rating (dB SPL per mW) separately to judge how loud a given power figure will actually sound.
Worked example — 2 V RMS into 32 Ω headphones
Enter 2 into Output voltage (V) and 32 into Headphone impedance (Ω) — a common 32 Ω headphone driven by a source outputting 2 V RMS. Power delivered reads (2² ÷ 32) × 1000 = (4 ÷ 32) × 1000 = 0.125 × 1000 = 125 mW.
That 125 mW figure is comfortably more than most 32 Ω consumer headphones need to reach a loud listening level, since typical sensitivity ratings mean only a handful of milliwatts are required for normal listening volume — which is exactly why an ordinary phone or laptop output drives this kind of headphone without any trouble.
Questions
Why do both voltage and impedance matter for headphone loudness?
Because power delivered — which drives loudness along with the headphone's own sensitivity — depends on both together: power rises with the square of voltage but falls as impedance rises. A source with plenty of voltage can still deliver little power into a very high-impedance headphone, and a low-impedance headphone can draw plenty of power even from a modest voltage, so neither number alone tells the full story.
Why do some headphones need a dedicated amplifier?
High-impedance headphones (often 250 Ω and up, common on studio and professional models) need more voltage to reach the same power level as a low-impedance consumer headphone, and many phones, laptops and DACs simply don't output enough voltage on their own. A dedicated headphone amplifier exists mainly to supply that higher voltage cleanly, restoring the power such headphones need.
Does more power always mean louder sound?
Not directly — loudness also depends on the headphone's sensitivity rating, expressed in decibels of sound pressure level per milliwatt (dB SPL/mW), which varies a lot between models. Two headphones fed the identical power figure from this calculator can sound quite different in volume if their sensitivity ratings differ, so power delivered and perceived loudness are related but not the same measurement.
What voltage does a typical phone or laptop headphone output supply?
It varies by device, but consumer headphone jacks commonly output on the order of 1 to 2 V RMS into a light load, with the actual figure depending on the device's internal amplifier design and dropping further under a heavier (lower-impedance) load. Check a specific device's published specifications for an exact figure rather than assuming a universal value.
Is milliwatts really the standard unit for headphone power specs?
Yes — headphone sensitivity is conventionally rated in decibels of sound pressure level per milliwatt (dB SPL/mW) rather than per watt, since headphones operate at power levels many times smaller than loudspeakers. That convention is exactly why this calculator's result is reported in milliwatts, matching the units used on headphone spec sheets.