Apexa Pro is built and maintained by one person. Every calculator is free, runs entirely in your browser, and needs no account or sign-up — and it will stay that way whether you donate or not.
The work behind it is real, though: designing each tool, checking the maths, writing the guides, and keeping the whole thing fast. If any of it has been useful to you and you'd like to help keep it going, you can support the project through Ko-fi. It's a small tip jar, not a subscription — give once, give nothing, either is completely fine.
One-time and secure, through Ko-fi. The site stays free for everyone either way.
Thank you for using the site. That already means a lot.