UART Baud Register Calculator (UBRR)
AVR UBRR value, actual baud and error percent — including the infamous 16 MHz / 115200 case and the U2X fix.
The infamous 16 MHz/115200 case: UBRR=8 gives −3.5 % error — right at the edge, which is why some Uno↔module pairs randomly drop bytes; U2X mode fixes it (UBRR=16, 2.1 %). “UART-friendly” crystals (7.3728, 11.0592, 14.7456 MHz) divide perfectly to every standard baud. The same divisor-and-error logic applies to STM32 (BRR) and ESP32 (CLKDIV).
UART Baud Register Calculator computes the AVR UBRR value, the real achieved baud and the error percentage — free, instant and private in your browser. AVR/Arduino developers chasing dropped serial bytes use it to skip the datasheet algebra: type your numbers, read the answer with the substituted formula shown step by step, and share an exact permalink of the calculation.
About UART Baud Register Calculator (UBRR)
UART Baud Register Calculator computes the AVR UBRR value, the real achieved baud and the error percentage using the standard engineering relation: UBRR = f(cpu)/(16·baud) − 1 (÷8 with U2X); keep error within ±2 %. Worked live: 16 MHz at 115200 gives UBRR 8 with −3.5 % error — marginal; U2X mode fixes it to 2.1 %. The result recalculates on every keystroke, the worked-example panel shows your numbers substituted into the formula, and the Copy permalink button encodes the inputs in the URL so a colleague opens exactly your calculation. Everything runs client-side — nothing you type leaves your device.
How to use UART Baud Register Calculator (UBRR)
- 1Enter your values — the tool starts with realistic defaults for this exact use case, so the worked example is meaningful immediately.
- 2Read the live result and the worked-example panel, which substitutes your numbers into the formula step by step.
- 3Adjust any input to compare scenarios, then use Copy result or Copy permalink to share the calculation.
Why use UART Baud Register Calculator (UBRR)?
- ✓Implements the real formula — UBRR = f(cpu)/(16·baud) − 1 (÷8 with U2X) — with the substitution shown, not a black box
- ✓Built for AVR/Arduino developers chasing dropped serial bytes
- ✓Copy result and permalink buttons — share the exact calculation in a README, forum answer or design review
- ✓100% free, no sign-up, runs entirely in your browser (works offline once loaded)
Frequently asked questions
How do you calculate uart baud register?+
The AVR UBRR value, the real achieved baud and the error percentage follows UBRR = f(cpu)/(16·baud) − 1 (÷8 with U2X); keep error within ±2 %. For example, 16 MHz at 115200 gives UBRR 8 with −3.5 % error — marginal; U2X mode fixes it to 2.1 %. The calculator applies the same relation and shows the substituted arithmetic so you can verify every step.
Why does serial work at 9600 but corrupt at 115200?+
Divider granularity: at 16 MHz, 9600 lands within 0.2 % but 115200 misses by 3.5 % in normal mode — both ends' errors add, and past ~4 % total the stop bit samples wrong. Enable U2X, drop to 76800/57600, or use a UART-friendly crystal.
What makes 14.7456 MHz a 'magic' crystal?+
It divides EXACTLY into every standard baud: 14745600/(16×115200) = 8 with zero remainder. The 7.3728 and 11.0592 MHz parts share the property. Designers pick them when serial integrity matters more than a rounder CPU clock.
Is the UART Baud Register Calculator free and private?+
Yes — completely free with no sign-up or usage limits, and it runs entirely in your browser: the values you enter are never uploaded or stored on a server.
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