I built free peptide calculators — PeptideCalculator.fit
I spent some time reviewing the peptide calculators that appear in search results and on peptide vendor sites. Most of them solve only one part of the process, and many return a syringe number without showing how mass, concentration, volume, and U-100 markings relate to each other.
I built this as an attempt to make that calculation path explicit:
vial mg ÷ liquid mL = concentration
target mg ÷ concentration = dose volume
dose volume × 100 = U-100 syringe units
It also has a reverse mode. If someone already knows the concentration and has a syringe reading, it converts the U-100 units back into mL, mg, and mcg. Reconstitution and direct mL/U-100 conversion are available as separate tools rather than being mixed into one large form.
The implementation uses Next.js and TypeScript. The arithmetic lives in small pure functions, calculations happen in the browser, and invalid inputs fail without producing a result. The UI also warns when a result exceeds the selected syringe capacity, is below one unit, or is larger than the total vial amount.
I deliberately avoided recommended doses and protocols. The site performs unit conversion and arithmetic only, with the formulas visible so results can be checked independently.
I’d be interested in feedback on the reverse-calculation flow, whether the terminology is understandable, and whether showing the formulas makes the result easier to verify.
Clear problem statement. A lot of these tools hide the math, so users cannot sanity-check the syringe number. Showing the three steps openly is the right call, especially since you are not giving dosing advice.
On reverse mode: that is probably the most useful part for returning users. People often remember "I drew to 12 units" more than the mg math. If reverse is buried, consider making it a first-class toggle next to the main calculator, not a separate afterthought.
Terminology: vial mg, concentration, dose volume, and U-100 units will make sense to people who already reconstitute. For first-timers, a one-line glossary under each field (what this number is on the vial / on the syringe) would reduce mistakes more than more features.
The capacity / below-one-unit / exceeds-vial warnings are excellent. Those are the exact failure modes that make bad calculators dangerous even when the arithmetic is "fine."
One small UX ask: keep the formula strip updating live as inputs change, so people see which step broke when a result disappears. That matches your "verify independently" goal.
Nice constraint on scope. Happy to poke the reverse flow if you want a second pair of eyes on a specific edge case.
Niche but genuinely useful — calculators like this often get built once out of personal necessity and end up helping way more people than expected. What was the original problem that made you build this?
Nice. Small calculators look simple from the outside, but the hard part is usually the assumptions around the number. I like when a calculator says what it is not handling, not just the final result. That makes it feel way more trustworthy.
The visual warnings for exceeding syringe capacity or total vial amount are a fantastic touch! It prevents user error without feeling restrictive. Keeping it purely focused on arithmetic without pushing protocols is a very smart approach. Are you planning to add a quick 'copy/share summary' button for users to save their math?
Showing the formulas instead of treating the calculator like a black box is a nice touch.
Being able to verify the result is probably just as valuable as getting it quickly.