Open any of the popular peptide calculators on the App Store. Type in a 5 mg BPC-157 vial, 2 mL of bacteriostatic water, and a target dose of 250 mcg. The answer comes back as 0.10 mL · 10 units. Clean. Confident.
Now run it for 300 mcg. The calculator returns 0.12 mL · 12 units. Still clean. Still confident. And · depending on the rounding strategy · it might be wrong by half a unit by the end of the vial.
The problem isn't the math. It's the silent rounding. And it's the reason power users on r/Peptides routinely keep a paper notebook with their own arithmetic. They've been burned.
The math, all of it
Reconstitution is a one-line equation:
concentration (mg/mL) = vial size (mg) / BAC water (mL)
dose volume (mL) = target dose (mg) / concentration (mg/mL)
syringe units (U-100) = dose volume (mL) × 100
For our BPC-157 example:
- 5 mg ÷ 2 mL = 2.5 mg/mL
- 0.250 mg ÷ 2.5 mg/mL = 0.10 mL
- 0.10 mL × 100 = 10 units on a U-100 syringe
So far so good. But notice: the answer to step 2 is exact. There's no rounding required. It just happens to land at 0.10 mL.
Now try 0.300 mg:
- 0.300 mg ÷ 2.5 mg/mL = 0.12 mL exactly
- 0.12 mL × 100 = 12 units exactly
Still clean. The trouble starts when your target dose isn't a clean multiple of your concentration.
Where rounding bites
Try a real-world recon: a 10 mg vial of Tirzepatide, 2 mL BAC water, target 2.5 mg/week.
- 10 ÷ 2 = 5 mg/mL
- 2.5 ÷ 5 = 0.5 mL = 50 units
Clean. Now bump the ladder: 2.5 → 5 → 7.5 → 10 → 12.5 → 15 mg/week.
At 5 mg, you need 1.0 mL. At 7.5 mg, you need 1.5 mL. At 15 mg, you need 3.0 mL · but your vial only holds 2 mL of reconstituted volume. You've outgrown the vial.
A calculator that rounds at 2 decimals will tell you 1.50 mL is fine. A calculator that shows you the formula · and the volume left in the vial · will warn you that you have 13 days of doses left, not 28.
This is why Regimio shows 4 decimal places of mL and tracks vial volume remaining in the same view. The rounding isn't wrong. The hiding is wrong.
The forward calculator vs the reverse
Most apps stop at the forward calculator: vial size + BAC water + target dose → answer.
Power users keep asking for the reverse:
"I want my doses to land on clean 10-unit increments. How much BAC water should I add?"
The math is the same equation rearranged:
BAC water (mL) = (vial size (mg) × syringe units desired) / (target dose (mg) × 100)
For a 5 mg BPC vial, target 250 mcg, target syringe = 10 units:
- (5 × 10) ÷ (0.250 × 100) = 50 ÷ 25 = 2.0 mL of BAC water
So you draw exactly 10 units per dose. The whole vial = 20 doses.
Now try target 200 mcg, target = 10 units on the same vial:
- (5 × 10) ÷ (0.200 × 100) = 50 ÷ 20 = 2.5 mL of BAC water
Two and a half mL fits in most peptide vials. Now your 10-unit draw on a U-100 syringe is exactly 200 mcg. Clean dosing. No squinting at the barrel.
The reverse calculator is the feature that converts power users from spreadsheet-on-the-side to just-use-the-app. Regimio puts it next to the forward calculator. Tap to toggle.
U-100 vs U-50
A U-100 syringe is scaled so 100 units = 1 mL. A U-50 is scaled so 50 units = 0.5 mL. Same physical volume per unit; just different marks on the barrel.
Some compounds (like NAD+ at clinic doses) are easier to draw on a U-50 because the marks are bigger. Some peptide users prefer U-100 because they're stocked. The calculator should know which you're using and show the answer in the right unit count · without asking you to switch screens.
In Regimio, the syringe type is a setting per compound, and the math is shown both ways at the bottom of the recon sheet:
BPC-157 · 250 mcg dose
0.1000 mL | 10 units (U-100) | 5 units (U-50)
The BAC water 28-day rule
Bacteriostatic water is sterile water with 0.9% benzyl alcohol. The benzyl alcohol prevents bacterial growth, but only for about 28 days after the vial is first punctured. After that, the manufacturer's stability data goes out the window, and you're rolling the dice on contamination.
This is the single most important reminder Regimio fires · not in the calculator, but in the vial card on your home screen:
BPC-157 vial · day 9 of 28 · 20 doses remaining
And as a local notification 3 days before expiry. If you've ever found a half-used vial in your fridge and squinted at the punch date in Sharpie, you'll know why this matters.
What no rounding actually looks like
Most calculators show:
Draw: 0.10 mL for a 250 mcg dose.
Regimio shows:
5 mg ÷ 2 mL = 2.5000 mg/mL · 0.250 mg ÷ 2.5 = 0.1000 mL · ×100 = 10 units (U-100)
The math is visible. The reasoning is visible. The 4-decimal precision is visible.
When you switch to a target that doesn't divide cleanly · say, 137 mcg of MK-677 in a 5 mg / 2 mL vial · you get:
0.137 mg ÷ 2.5 mg/mL = 0.0548 mL · ×100 = 5.48 units (U-100)
Five and a half units. Not "round to 5" or "round to 6." The truth. Then you decide which way to round, manually, on your syringe.
Why this is the moat
There are 22 apps in this category. Half of them have a reconstitution calculator. Almost all of them round silently. Power users notice, and they distrust everything else in the app after the first rounded answer.
Reconstitution is the easiest place to win user trust. It's also the easiest place to lose it. We chose 4 decimal places and the visible formula because once you trust the math, you trust everything else.
"Tired of confusing math, online calculators with ads, or complex spreadsheets." · App Store review of PepCalc, Apr 2025
That sentence wrote half our spec. The other half is in our features page and the v0 build spec.
Regimio's reconstitution engine ships at v0. No rounding. Forward and reverse. Embedded in the logger · so the math happens at the moment of the dose, not in a separate "calculator" tab nobody opens twice.
Try it in the private beta. And bring your worst recon edge case.