Diabetes Carb Counter Calculator

This calculator applies two long-standing rules of thumb used in diabetes education to turn a patient's own total daily insulin dose (TDD -- basal plus all bolus doses) into personalized carb-counting and correction-dosing ratios, then uses those ratios to size an actual meal dose. The "500 rule" estimates the insulin-to-carbohydrate ratio (ICR) as 500 divided by TDD, giving the number of grams of carbohydrate that one unit of rapid-acting insulin is expected to cover -- a patient on a lower total daily dose gets a wider ratio (each unit covers more carbs) because the 500 rule assumes their overall insulin sensitivity is higher. The "1800 rule" works the same way for correction dosing, estimating the insulin sensitivity factor (ISF) as 1800 divided by TDD for rapid-acting insulin -- the number of mg/dL that one unit is expected to lower blood glucose. The total bolus recommendation adds together a carb dose (meal carbohydrates divided by ICR) and, only when current blood glucose is above the entered target, a correction dose (the excess glucose divided by ISF); when blood glucose is at or below target, no correction is added.

The carb dose itself, however, is calculated unconditionally regardless of how low the entered blood glucose is -- it does not check for hypoglycemia the way the correction dose checks for hyperglycemia. A pre-meal reading below about 70 mg/dL calls for treating the low first (per your care team's hypoglycemia protocol), not dosing insulin for the meal on top of it, which is why this calculator flags a Low Glucose Warning at that threshold rather than silently returning a full carb-bolus recommendation. Both the 500 and 1800 rules are population-derived starting points from diabetes education literature, not measurements of an individual patient's actual insulin sensitivity -- real-world ratios are usually refined over time by a diabetes care team based on observed glucose response, and can differ meaningfully from the rule-of-thumb value, especially early in insulin pump therapy or during illness, pregnancy, or major activity changes. This tool is an educational illustration of how the ratios are derived and combined, not a substitute for ratios set by an endocrinologist or certified diabetes educator.