Back of the envelope

Napkin Calc

This is a formula calculator for rough system estimates. You write a short math expression using known speeds and prices. It is not a chatbot and the left box is not a question search—it only filters the list of building-block names.

  1. Pick a building block on the left (a speed, a delay, or a monthly price).
  2. Build a formula on the right, or click a block to insert its name.
  3. Read the answer under the formula. Green text is the estimate.

2. Your formula

3. Answer

What you can use in a formula

Math
+ - * / ^ and parentheses ( )
Numbers
10, 10k, 100_000, 1.5e6, 2M, 3G
Sizes
B KiB MiB GiB TiB (binary), and KB MB GB TB (for prices)
Time
ns us ms s min h day month year
Rates
rps / qps (requests per second), and speeds like MiB/s GiB/s
Building-block names
Any short id from the left list: speeds/delays (seq_mem, syscall), generic prices (cpu, logs), cloud prices (aws_cpu, gcp_vm, azure_egress), compression (html)
Extra columns
.latency .throughput on speeds; .spot .commit on prices. Example: cpu.spot, seq_mem.latency
Common shapes
size / speed → time · count * delay → time · amount * price → monthly cost · rps * bytes * price → monthly cost · a + b only when units match
Worked examples
1 GiB / seq_mem · 10k * syscall · 4 * aws_cpu + 16 GB * aws_memory · aws_vm · 100k rps * 500 B * logs · 1 TiB * aws_egress

Glossary

Common words used in the formulas above.