The 500 Rule and 1800 Rule are standard methods for estimating starting insulin-to-carbohydrate ratios and correction factors based on Total Daily Dose (TDD). These are starting points — individual responses require ongoing adjustment.
Calculate Insulin Ratios
Sum of all basal + bolus insulin units taken per day
How the Insulin Dose Calculator Works
This insulin dose calculator uses two standard clinical formulas — the 500 Rule and 1800 Rule — to estimate starting insulin ratios from Total Daily Dose (TDD). These formulas are widely used in diabetes care for establishing initial basal-bolus ratios when switching insulin regimens.
The 500 Rule (Insulin-to-Carb Ratio)
Dividing 500 by your TDD gives the grams of carbohydrates covered by 1 unit of rapid-acting insulin. A TDD of 40 units: 500 ÷ 40 = 12.5, meaning 1 unit covers approximately 12.5g carbs. To dose for 60g of carbs: 60 ÷ 12.5 = ~5 units.
The 1800 Rule (Correction Factor)
Dividing 1800 by your TDD gives the mg/dL blood sugar drop per 1 unit of rapid-acting insulin. A TDD of 40 units: 1800 ÷ 40 = 45, meaning 1 unit drops blood sugar by approximately 45 mg/dL. If target is 120 mg/dL and current reading is 210 mg/dL: correction = (210 - 120) ÷ 45 = 2 units.
Frequently Asked Questions
Is this insulin calculator free?
Yes, completely free with no signup required.
Should I use this calculator to adjust my insulin doses?
Only use this calculator in consultation with your healthcare provider. Insulin dosing is highly individual. This tool provides reference calculations that your provider may use to establish starting ratios, but actual doses must be determined and adjusted by your endocrinologist or diabetes care team.
What is the 500 Rule?
The 500 Rule estimates your insulin-to-carbohydrate ratio: 500 ÷ Total Daily Dose (TDD) = grams of carbohydrates covered by 1 unit of insulin. For example, TDD of 50 units: 500 ÷ 50 = 10, meaning 1 unit covers 10g of carbohydrates.
What is the 1800 Rule (or 1700 Rule)?
The 1800 Rule estimates your insulin sensitivity (correction factor): 1800 ÷ TDD = mg/dL drop per 1 unit of insulin. Some clinicians use 1700 for regular insulin. Example: TDD of 50: 1800 ÷ 50 = 36, meaning 1 unit drops blood sugar by approximately 36 mg/dL.
What is Total Daily Dose (TDD)?
TDD is the sum of all insulin units taken in 24 hours: basal (background) insulin + all bolus (mealtime and correction) insulin doses combined over a typical day. Your endocrinologist or diabetes educator can help you calculate an accurate TDD from your insulin logs.