Why do two 80 dB sources make 83 dB and not 160 dB?

Decibels are logarithmic, so you cannot add the numbers directly. Sound power adds linearly, so you convert each level back to a power value, sum them, then convert back: L = 10·log10(10^8.0 + 10^8.0). Two equal sources double the total power, and doubling power is +3.01 dB. So 80 + 80 dB combine to 83 dB. Three equal sources add about 4.77 dB, ten equal sources add exactly 10 dB.

When do I use 10·log versus 20·log?

Use 10·log10 for a power or intensity ratio, such as acoustic power, electrical power in watts, or sound intensity. Use 20·log10 for an amplitude ratio, such as voltage, sound pressure, or current, because power is proportional to amplitude squared and the square pulls out as a factor of two. A voltage ratio of 2 is 6.02 dB, but a power ratio of 2 is only 3.01 dB. Picking the wrong one doubles or halves your answer.

What does adding 3 dB or 10 dB actually mean?

Adding 3 dB doubles the power: a ratio of 2 is 10·log10(2) = 3.01 dB. Adding 10 dB is ten times the power, since 10·log10(10) = 10 dB exactly. For loudness as the ear judges it, a rough rule is that +10 dB sounds about twice as loud, while +3 dB is a clear but smaller jump. The example list in the tool lets you check these against real combinations.

Does combining levels need each source to be equal?

No. Enter any mix of levels and the tool sums them correctly. When one source is much louder, it dominates: 85 dB plus 60 dB combine to about 85.01 dB, because the quiet 60 dB source adds almost no power. This is why a single loud machine sets the room level and a second quiet one barely moves it. Equal sources are the case where the +3 dB rule applies cleanly.

Is sound pressure level the same as the dB this tool computes?

The addition math is identical for sound pressure level (SPL) in dB, because SPL is already referenced to 20 micropascals and combines on the same logarithmic scale. Enter your measured SPL figures and the combined value is the SPL you would read with both sources running, assuming the sources are uncorrelated. Pure tones at the same frequency can interfere and need a different treatment, but broadband noise like traffic or machinery adds as this tool does.

Are my numbers sent anywhere?

No. Every calculation is plain JavaScript running in your browser tab. The levels, ratios, and mode are stored only in the page URL so a shared link reopens the same calculation, and nothing is uploaded or logged. If a calculation is confidential, use the copy button and paste the text instead of sharing the URL, since the query string travels with the link.

Predict the noise of two machines running together

A workshop has one machine at 85 dB and you are about to add a second at 82 dB. Type both into the add list and the tool tells you the combined level is about 86.8 dB, not 167. That single figure decides whether the room crosses an 85 dB exposure threshold and whether hearing protection becomes mandatory, so you can plan the layout before either machine is wired in.

Read or set an amplifier gain from a spec sheet

A spec lists a stage as 20 dB of voltage gain and you want the linear factor. Switch to ratio mode, choose amplitude, type 20 dB, and read a 10× voltage ratio. Going the other way, an output that is three times the input voltage is 9.54 dB. The 20·log path keeps you from accidentally using the 10·log power formula and halving the number.

Estimate a quiet room's level from several small sources

An office has HVAC at 45 dB, a fan at 40 dB, and a laptop at 35 dB. Add all three and the tool returns about 46.7 dB, dominated by the HVAC. That tells you silencing the laptop changes almost nothing while quieting the HVAC matters, so you spend the noise-control budget where it actually moves the reading.

Teach why decibels are logarithmic

A physics or audio student needs to see, not just be told, that decibels do not add arithmetically. Have them combine two 70 dB sources to get 73 dB, then three to get about 74.8 dB, then ten to get exactly 80 dB. The pattern of diminishing additions makes the 10·log10 relationship concrete, and the shareable URL lets the instructor hand out a worked example.

Decibel Calculator — Add dB, Convert Ratio ⇄ Decibels

Add sound levels, convert ratio ⇄ dB (power 10·log, amplitude 20·log), and read a common-sound reference table — browser-only

Runs locally
Category Calculator
Best for Getting a realistic range before a purchase, plan, workout, or schedule decision.

Enter each source level in dB, one per line or comma separated

Combined level

2 sources

Adding 3 dB doubles the power; adding 10 dB is 10× the power.

Common sound levels

Click a row to copy the dB value; in Add mode it appends to the list

What this tool does

A decibel calculator that does the three things people actually search for. First, add decibels: drop in several source levels and it combines them on a logarithmic scale, so two 80 dB machines read 83 dB, not 160. The math is L = 10·log10(Σ 10^(Li/10)), which is why doubling identical sources adds about 3 dB. Second, convert between a ratio and decibels in both directions: power ratios use 10·log10(P2/P1), amplitude or voltage ratios use 20·log10(V2/V1), and you can go either way to size an amplifier gain or read a spec sheet. Third, a reference table of common sounds from a whisper at 30 dB to a jet engine, with one-click copy that also feeds the add list. Everything runs in your browser; the inputs and mode live in the URL so a shared link reopens the exact calculation.

Tool details

Input: Text + Numbers; The page exposes text boxes, numeric controls, file pickers, or structured inputs depending on the tool.
Output: Live result + Copy; The result area focuses on usable output, with copy, download, or preview actions when supported.
Privacy: Browser-side processing; The main tool logic does not call an external API, so inputs normally stay in the current tab.
Save / share: Shareable URL state; Key settings are encoded in the URL so another person can reopen the same setup.
Performance budget: Initial JS <= 9 KB; No WASM budget is declared, keeping the tool quick to open on mobile.
Best fit: Calculator · Developer; Category and role tags drive related tools, internal links, and quick fit checks.

How to use

1. Input

Paste or drop your content into the tool panel.
2. Process

Click the button. All processing is local in your browser.
3. Copy / Download

Copy the result or download to disk in one click.

How Decibel Calculator fits into your work

Use it for fast estimates, comparisons, and planning numbers before you make the final call.

Calculation jobs

Getting a realistic range before a purchase, plan, workout, or schedule decision.
Comparing scenarios by changing one input at a time.
Turning rough assumptions into a number you can discuss.

Calculation checks

Double-check units, dates, rates, and rounding assumptions.
Treat health, finance, tax, and legal outputs as planning aids, not professional advice.
Save the inputs that produced an important result so you can reproduce it later.

Good next steps

These links move the current task into a more complete workflow.

Real-world use cases

Predict the noise of two machines running together
A workshop has one machine at 85 dB and you are about to add a second at 82 dB. Type both into the add list and the tool tells you the combined level is about 86.8 dB, not 167. That single figure decides whether the room crosses an 85 dB exposure threshold and whether hearing protection becomes mandatory, so you can plan the layout before either machine is wired in.
Read or set an amplifier gain from a spec sheet
A spec lists a stage as 20 dB of voltage gain and you want the linear factor. Switch to ratio mode, choose amplitude, type 20 dB, and read a 10× voltage ratio. Going the other way, an output that is three times the input voltage is 9.54 dB. The 20·log path keeps you from accidentally using the 10·log power formula and halving the number.
Estimate a quiet room's level from several small sources
An office has HVAC at 45 dB, a fan at 40 dB, and a laptop at 35 dB. Add all three and the tool returns about 46.7 dB, dominated by the HVAC. That tells you silencing the laptop changes almost nothing while quieting the HVAC matters, so you spend the noise-control budget where it actually moves the reading.
Teach why decibels are logarithmic
A physics or audio student needs to see, not just be told, that decibels do not add arithmetically. Have them combine two 70 dB sources to get 73 dB, then three to get about 74.8 dB, then ten to get exactly 80 dB. The pattern of diminishing additions makes the 10·log10 relationship concrete, and the shareable URL lets the instructor hand out a worked example.

Common pitfalls

Adding decibels arithmetically. Two 80 dB sources are not 160 dB; they combine to 83 dB because the scale is logarithmic. Always sum the power values first, then convert back, which is exactly what the add mode does.
Using 10·log for a voltage or pressure ratio. Amplitude ratios need 20·log10, so a voltage ratio of 2 is 6.02 dB, not 3.01 dB. Picking the power formula for an amplitude quantity halves your decibel figure.
Assuming a second quiet source noticeably raises the level. A 60 dB source added to an 85 dB source lifts it by about 0.01 dB. The loudest source dominates, so chasing the quiet contributors wastes effort.

Privacy

Every calculation here — the logarithmic addition, the ratio conversions, and the reference table — is plain JavaScript that runs in your browser tab. No level, ratio, or result is uploaded or logged. The one caveat: the shareable URL encodes your inputs and mode in the query string, so a link pasted into chat records those numbers in the recipient server's access log. For a confidential figure, use the copy button and paste the text rather than sharing the URL.

FAQ

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Decibel Calculator — Add dB, Convert Ratio ⇄ Decibels

What this tool does

Tool details

How to use

1. Input

2. Process

3. Copy / Download

How Decibel Calculator fits into your work

Calculation jobs

Calculation checks

Good next steps

Real-world use cases

Predict the noise of two machines running together

Read or set an amplifier gain from a spec sheet

Estimate a quiet room's level from several small sources

Teach why decibels are logarithmic

Common pitfalls

Privacy

FAQ

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