How do you convert ppm to percent?

1 percent equals 10,000 ppm, because ppm means parts per million and percent means parts per hundred. Divide the ppm value by 10,000: 1,000 ppm CO₂ is 0.1 percent by volume, 400 ppm is 0.04 percent. The other way round, multiply a percentage by 10,000 to get ppm.