Insulin to carb ratio calculator: the 500 and 1800 rules explained
For people already working with their diabetes team. Follow your care team’s plan; this page gives rule-of-thumb estimates, not doses, and is not medical advice.
This insulin to carb ratio calculator applies the published rules of thumb that diabetes teams use as a first guess for two pump and bolus-calculator settings: the insulin-to-carbohydrate ratio (how many grams of carbohydrate one unit of insulin covers) and the correction factor or insulin sensitivity factor (how far one unit lowers glucose). Both come from your own total daily dose of insulin, which you have to enter — there is no default.
It shows the 500 and 450 rules, the 1800, 1700, 1500 and 100 rules, and the weight-based formula from Davidson and colleagues side by side, in mg/dL and mmol/L, plus a printable carb ratio chart. Use it to understand where the numbers on your settings came from or to prepare questions for your next appointment.
How to use the carb ratio calculator
- Choose your mealtime insulin: Rapid-acting (such as lispro, aspart or glulisine) or Regular human insulin.
- Pick mg/dL or mmol/L for glucose.
- Enter your total daily dose: all basal and bolus insulin in a typical day, averaged over several days. Add your weight for the Davidson formula if you like.
- Read the carb ratio and correction factor, compare all the rules in the table, and copy the summary to discuss with your diabetes team.
Insulin-to-Carb Ratio & Correction Factor Calculator features
Every common rule
500 and 450 for carb ratio; 1800, 1700, 1500 and 100 for correction, labelled by insulin type.
Both glucose units
Correction factors in mg/dL and mmol/L (1 mmol/L = 18.016 mg/dL).
Carb ratio chart
Ratios for total daily doses from 15 to 100 units, with your row highlighted.
Safety first
No default dose, plausibility checks, and no dose is ever calculated.
Sourced
Each formula is named with its published source.
Carb ratio formula and correction factor formula
Carb ratio (500 rule): 500 ÷ total daily dose = grams of carbohydrate covered by 1 unit of rapid-acting insulin. With a total daily dose of 50 units, 500 ÷ 50 = 10, a ratio of 1 unit per 10 g. Many teams use 450 for regular insulin.
Correction factor (1800 rule): 1800 ÷ total daily dose = how many mg/dL one unit of rapid-acting insulin lowers glucose; 1800 ÷ 50 = 36 mg/dL. In mmol/L the “100 rule” gives 100 ÷ 50 = 2.0 mmol/L. The 1500 rule (about 83 in mmol/L) is used with regular insulin.
Davidson and colleagues (2008) analysed well-controlled pump users and proposed a correction factor of 1700 ÷ TDD and a carb ratio of 2.8 × body weight in pounds ÷ TDD (Endocrine Practice 14:1095–1101). Walsh, Roberts and Bailey (2011) describe how bolus calculators combine these settings with active insulin and a target.
Why these are only starting points
- They were developed mainly in adults with type 1 diabetes. Hanas and Adolfsson (2017) found that well-controlled young children needed more meal insulin than the 500 rule predicts, especially at breakfast, and less correction insulin than the 100 rule; Hegab (2019) found 300–375 ÷ TDD fitted children on injections better.
- Real ratios usually differ between breakfast, lunch and dinner, and shift with exercise, illness, menstrual cycle, growth and weight change.
- In type 2 diabetes with insulin resistance, or with very high daily doses, the rules may not fit at all.
- Your team refines the numbers using glucose records or CGM data, testing one setting at a time.
Insulin-to-Carb Ratio & Correction Factor Calculator FAQ
How do I calculate my insulin to carb ratio?
The 500 rule divides 500 by your total daily dose of insulin: 40 units a day gives 12.5 g of carbohydrate per unit. It is only a first estimate; your diabetes team adjusts it from your glucose records.
What is the 1800 rule for correction factor?
1800 divided by the total daily dose estimates how many mg/dL one unit of rapid-acting insulin lowers blood glucose. The mmol/L version is the 100 rule.
Is this an insulin dose calculator?
No. It estimates the two settings that bolus calculators use, and only as rule-of-thumb starting points. It never tells you how much insulin to take; that plan comes from your diabetes care team.
What should I enter as total daily dose?
All the insulin you take in a typical day — long-acting or pump basal plus every bolus — averaged over several days.
Why is my real carb ratio different?
Because everyone’s insulin needs are different and change through the day. Teams use these rules to start, then adjust with records.
Where can I log my readings for my team?
The blood sugar log records glucose in mg/dL or mmol/L, and the mg/dL to mmol/L converter converts single values.