Quick answer: Take the price after the coupon, divide by the milligrams in the vial, and add shipping before you divide if you want the true cost. The lowest price per mg wins the price comparison.
Why the biggest coupon can lose
A bigger percentage off a higher price can still cost more. In an example with made-up vendors, Vendor A takes 20% off $60 for a 10 mg vial: $48, or $4.80 per mg. Vendor B takes 10% off $45 for a 10 mg vial: $40.50, or $4.05 per mg. The smaller coupon wins.
The formula
Price after coupon = regular price x (1 - discount / 100).
Price per mg = price after coupon / vial size in mg.
With shipping = (price after coupon + shipping) / vial size in mg.
Our price per mg calculator does this for two vendors at once.
Seven things to line up
- The regular price before any code.
- The coupon and what it applies to.
- The vial size in mg, in the same unit for every vendor.
- The price per mg.
- Shipping and any free-shipping threshold.
- Whether a certificate of analysis exists for your lot.
- The product size and lot.
What price per mg cannot tell you
It compares cost only. It says nothing about purity or identity. For that, read the certificate of analysis for your specific lot. See our guide to reading a COA.
FAQ
Should I include shipping?
Yes, if you want the real total. Shipping can reverse which vendor is cheapest.
Are the example vendors real?
No. They are invented to show the arithmetic.