The is a classic logarithmic display driver used to power LED bar graphs for audio levels and signal strength. While the original LM3915 datasheet from Texas Instruments (formerly National Semiconductor) provides the fundamental formulas, modern "updated" calculators make the design process much faster.
Modern hobbyists often use 5V USB power or 3.7V LiPo batteries instead of the classic 9V/12V rails. Updated calculators quickly find the exact resistor ratios to keep the 3dB-per-step logarithmic scale accurate at these lower voltages. lm3915 calculator updated
You need less than 1% error, are using a non-standard supply voltage (<5V or >25V), or need to cascade multiple ICs. The is a classic logarithmic display driver used