Know what you're checking
The calculation, explained.
A visual result should come with an explanation. Here is the arithmetic behind Dosii, what the display rounds, and what a calculator cannot tell you.
By Dosii · Updated September 10, 2026
Three quantities. Different meanings.
- Mass: mg and mcg
- 1 mg equals 1,000 mcg. These describe an amount of substance.
- Volume: mL
- To convert an amount into a volume, the calculation also needs a concentration.
- Syringe scale
- A scale marking describes volume for a specified syringe. It does not represent a universal amount of a substance.
How the arithmetic works
For the standard calculation, Dosii converts the vial amount to mcg, then divides it by the entered water volume to calculate a concentration. It divides the intended amount by that concentration to calculate a volume.
mg × 1,000 = mcg
amount ÷ volume = concentration
intended amount ÷ concentration = calculated volume
For Dosii's supported U-100 syringe scales, one scale unit represents 0.01 mL. Syringe capacity and substance amount are different quantities. The selected scale must match the device being considered.
Follow the numbers
One calculation. Two rounded displays.
This hypothetical example explains the standard arithmetic with compensation off. These made-up quantities are not a dose or preparation recommendation.
- Vial amount
- 10 mg
- Entered liquid volume
- 2 mL
- Amount to calculate
- 275 mcg
- Convert the vial amount: 10 mg × 1,000 = 10,000 mcg.
- Calculate concentration: 10,000 mcg ÷ 2 mL = 5,000 mcg/mL.
- Calculate volume: 275 mcg ÷ 5,000 mcg/mL = 0.055 mL.
- For the selected U-100 scale: 0.055 mL ÷ 0.01 mL per unit = 5.5 scale units.
- Exact calculated volume
- 0.055 mL
- Volume shown to two decimal places
- 0.06 mL
- Units shown to the nearest half unit
- 5.5 units
Converting the rounded 0.06 mL display back into units gives 6, rather than 5.5. That happens because the volume display has already lost precision. Start from the original inputs when checking a result; converting a rounded display again does not recover the exact calculation.
This example describes numeric formatting. It does not establish the accuracy of a physical scale or tell you which mark to use.
Why mg cannot convert straight to syringe units
Concentration connects mass to volume. In a separate hypothetical comparison, 1 mg at 5 mg/mL occupies 0.2 mL; 1 mg at 10 mg/mL occupies 0.1 mL. The mass is the same, but the volume differs.
If two calculators disagree, first compare the mass units, concentration inputs and selected scale. Then compare their rounding conventions. A different displayed number alone does not show which calculator is correct.
For the mass-prefix relationship, see NIST's SI prefix reference. It supports the unit conversion, not a clinical use or an endorsement of Dosii.
Rounding deserves attention
The standard calculation retains an exact decimal result internally. Its numeric syringe-unit result rounds to the nearest half unit; its volume result rounds to two decimal places. A rounded display is not a claim that a physical syringe can measure that volume precisely.
Very small values can round to zero. Check the original inputs, units and applicable warnings. The arithmetic assumes the entered quantities and selected settings are correct; it cannot verify the contents of a vial, a concentration, or a physical measuring device.
Calculation code checks cover unit conversion, concentration, syringe-unit rounding and fractional quantities. Automated checks are not independent clinical validation.
Private calculations. Clear boundaries.
The iPhone app calculates and stores calculator values locally, without an account or app analytics. This marketing website uses separate visitor and App Store click measurement. Calculator values are not sent to this website or Meta. Read the privacy details.
Dosii is a mathematical reference. It does not select a dose, establish whether a substance is appropriate, or provide preparation, administration or treatment instructions.
A question about the app?
Contact Dosii support with the app version and a description of the screen or issue. Use made-up example values and omit personal health information. Support is for the app and its interface, not medical advice.
See the app screens →Dosii is available for iPhone for $9.99 once in the US store. No account or subscription.