Improved convertUnit accuracy and precision

jakeboone02 · jakeboone02 · commit e533187b1a0e · 2026-01-30T07:23:04.000-07:00
diff --git a/CHANGELOG.md b/CHANGELOG.md
@@ -10,6 +10,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 ### Fixed
 
 - `convertUnit` options `fromSystem` and `toSystem` are now case-insensitive.
+- `convertUnit` rounds results to 6 decimal places for more accurate conversions. Updated conversion factors with more precise values.
 
 ## [v2.0.0] - 2026-01-29
 
diff --git a/src/constants.ts b/src/constants.ts
@@ -252,14 +252,14 @@ export const unitsOfMeasure: UnitOfMeasureDefinitions = {
     plural: 'ounces',
     alternates: ['oz.'] satisfies string[],
     type: 'mass',
-    conversionFactor: 28.3495,
+    conversionFactor: 28.349523,
   },
   pound: {
     short: 'lb',
     plural: 'pounds',
     alternates: ['lb.', 'lbs', 'lbs.'] satisfies string[],
     type: 'mass',
-    conversionFactor: 453.592,
+    conversionFactor: 453.59237,
   },
 
   // Volume units (conversion factor in ml)
@@ -268,7 +268,7 @@ export const unitsOfMeasure: UnitOfMeasureDefinitions = {
     plural: 'cups',
     alternates: ['c.', 'C'] satisfies string[],
     type: 'volume',
-    conversionFactor: { us: 236.588, imperial: 284.131, metric: 250 },
+    conversionFactor: { us: 236.58824, imperial: 284.13063, metric: 250 },
   },
   deciliter: {
     short: 'dl',
@@ -295,14 +295,14 @@ export const unitsOfMeasure: UnitOfMeasureDefinitions = {
       'fluid-oz',
     ] satisfies string[],
     type: 'volume',
-    conversionFactor: { us: 29.5735, imperial: 28.4131 },
+    conversionFactor: { us: 29.57353, imperial: 28.413063 },
   },
   gallon: {
     short: 'gal',
     plural: 'gallons',
     alternates: ['gal.'] satisfies string[],
     type: 'volume',
-    conversionFactor: { us: 3785.41, imperial: 4546.09 },
+    conversionFactor: { us: 3785.4118, imperial: 4546.09 },
   },
   liter: {
     short: 'l',
@@ -323,28 +323,28 @@ export const unitsOfMeasure: UnitOfMeasureDefinitions = {
     plural: 'pints',
     alternates: ['pt.'] satisfies string[],
     type: 'volume',
-    conversionFactor: { us: 473.176, imperial: 568.261 },
+    conversionFactor: { us: 473.17647, imperial: 568.26125 },
   },
   quart: {
     short: 'qt',
     plural: 'quarts',
     alternates: ['qt.', 'qts', 'qts.'] satisfies string[],
     type: 'volume',
-    conversionFactor: { us: 946.353, imperial: 1136.52 },
+    conversionFactor: { us: 946.35295, imperial: 1136.5225 },
   },
   tablespoon: {
     short: 'tbsp',
     plural: 'tablespoons',
     alternates: ['tbsp.', 'T', 'Tbsp.', 'Tbsp', 'tablespoonful'] satisfies string[],
     type: 'volume',
-    conversionFactor: { us: 14.787, imperial: 17.758, metric: 15 },
+    conversionFactor: { us: 14.786765, imperial: 15, metric: 15 },
   },
   teaspoon: {
     short: 'tsp',
     plural: 'teaspoons',
     alternates: ['tsp.', 't', 'teaspoonful'] satisfies string[],
     type: 'volume',
-    conversionFactor: { us: 4.929, imperial: 5.919, metric: 5 },
+    conversionFactor: { us: 4.9289216, imperial: 5, metric: 5 },
   },
 
   // Volume units without conversion factor (imprecise)
diff --git a/src/convertUnit.test.ts b/src/convertUnit.test.ts
@@ -93,17 +93,17 @@ describe('volume conversions', () => {
 
   test('tablespoon to teaspoon (US)', () => {
     const result = convertUnit(1, 'tablespoon', 'teaspoon', { fromSystem: 'us' });
-    expect(result).toBeCloseTo(3, 0);
+    expect(result).toBe(3);
   });
 
   test('pint to cup (US)', () => {
     const result = convertUnit(1, 'pint', 'cup', { fromSystem: 'us' });
-    expect(result).toBeCloseTo(2, 0);
+    expect(result).toBe(2);
   });
 
   test('quart to pint (US)', () => {
     const result = convertUnit(1, 'quart', 'pint', { fromSystem: 'us' });
-    expect(result).toBeCloseTo(2, 0);
+    expect(result).toBe(2);
   });
 
   test('fluid ounce to milliliter (US)', () => {
@@ -142,12 +142,12 @@ describe('length conversions', () => {
 
   test('foot to inch', () => {
     const result = convertUnit(1, 'foot', 'inch');
-    expect(result).toBeCloseTo(12, 0);
+    expect(result).toBe(12);
   });
 
   test('yard to foot', () => {
     const result = convertUnit(1, 'yard', 'foot');
-    expect(result).toBeCloseTo(3, 0);
+    expect(result).toBe(3);
   });
 
   test('meter to centimeter', () => {
@@ -187,12 +187,12 @@ describe('fromSystem/toSystem relationships', () => {
   describe('same unit conversions', () => {
     test('cup US to cup US returns 1', () => {
       const result = convertUnit(1, 'cup', 'cup', { fromSystem: 'us', toSystem: 'us' });
-      expect(result).toBeCloseTo(1, 5);
+      expect(result).toBe(1);
     });
 
     test('cup Imperial to cup Imperial returns 1', () => {
       const result = convertUnit(1, 'cup', 'cup', { fromSystem: 'imperial', toSystem: 'imperial' });
-      expect(result).toBeCloseTo(1, 5);
+      expect(result).toBe(1);
     });
 
     test('cup US to cup Imperial', () => {
@@ -208,12 +208,12 @@ describe('fromSystem/toSystem relationships', () => {
     });
 
     test('tablespoon US to tablespoon Imperial', () => {
-      // US tbsp (14.787 ml) to Imperial tbsp (17.758 ml)
+      // US tbsp (14.786765 ml) to Imperial tbsp (15 ml)
       const result = convertUnit(1, 'tablespoon', 'tablespoon', {
         fromSystem: 'us',
         toSystem: 'imperial',
       });
-      expect(result).toBeCloseTo(14.787 / 17.758, 4);
+      expect(result).toBeCloseTo(14.786765 / 15, 4);
     });
 
     test('gallon Imperial to gallon US', () => {
@@ -248,16 +248,16 @@ describe('fromSystem/toSystem relationships', () => {
         fromSystem: 'metric',
         toSystem: 'metric',
       });
-      expect(result).toBeCloseTo(1, 5);
+      expect(result).toBe(1);
     });
 
     test('tablespoon Imperial to tablespoon metric', () => {
-      // Imperial tbsp (17.758 ml) to metric tbsp (15 ml)
+      // Imperial tbsp (15 ml) to metric tbsp (15 ml)
       const result = convertUnit(1, 'tablespoon', 'tablespoon', {
         fromSystem: 'imperial',
         toSystem: 'metric',
       });
-      expect(result).toBeCloseTo(17.758 / 15, 4);
+      expect(result).toBeCloseTo(15 / 15, 4);
     });
   });
 
@@ -276,12 +276,12 @@ describe('fromSystem/toSystem relationships', () => {
     });
 
     test('US tablespoon to Imperial teaspoon', () => {
-      // US tbsp (14.787 ml) to Imperial tsp (5.919 ml)
+      // US tbsp (14.786765 ml) to Imperial tsp (5 ml)
       const result = convertUnit(1, 'tablespoon', 'teaspoon', {
         fromSystem: 'us',
         toSystem: 'imperial',
       });
-      expect(result).toBeCloseTo(14.787 / 5.919, 3);
+      expect(result).toBeCloseTo(14.786765 / 5, 3);
     });
 
     test('Imperial pint to US cup', () => {
@@ -309,12 +309,12 @@ describe('fromSystem/toSystem relationships', () => {
   describe('units without multi-system factors', () => {
     test('gram to gram returns 1 regardless of system', () => {
       const result = convertUnit(1, 'gram', 'gram', { fromSystem: 'us', toSystem: 'imperial' });
-      expect(result).toBeCloseTo(1, 5);
+      expect(result).toBe(1);
     });
 
     test('liter to liter returns 1 regardless of system', () => {
       const result = convertUnit(1, 'liter', 'liter', { fromSystem: 'imperial', toSystem: 'us' });
-      expect(result).toBeCloseTo(1, 5);
+      expect(result).toBe(1);
     });
 
     test('pound to kilogram same regardless of system', () => {
@@ -360,7 +360,7 @@ describe('with additionalUOMs', () => {
 describe('unit spelling variations', () => {
   test('accepts short form units', () => {
     expect(convertUnit(1, 'c', 'ml')).toBeCloseTo(236.588, 2);
-    expect(convertUnit(1, 'tbsp', 'tsp')).toBeCloseTo(3, 0);
+    expect(convertUnit(1, 'tbsp', 'tsp')).toBe(3);
     expect(convertUnit(1, 'lb', 'g')).toBeCloseTo(453.592, 2);
   });
 
@@ -370,7 +370,7 @@ describe('unit spelling variations', () => {
   });
 
   test('accepts alternate spellings', () => {
-    expect(convertUnit(1, 'T', 'tsp')).toBeCloseTo(3, 0);
+    expect(convertUnit(1, 'T', 'tsp')).toBe(3);
     expect(convertUnit(1, 'lbs', 'g')).toBeCloseTo(453.592, 2);
   });
 
@@ -425,7 +425,7 @@ describe('fromSystem and toSystem case-insensitivity', () => {
       fromSystem: 'METRIC' as UnitSystem,
       toSystem: 'metric',
     });
-    expect(result).toBeCloseTo(1, 5);
+    expect(result).toBe(1);
   });
 
   test('mixed case produces same result as lowercase', () => {
diff --git a/src/convertUnit.ts b/src/convertUnit.ts
@@ -121,6 +121,11 @@ export const convertUnit = (
     return null;
   }
 
+  // Same unit with same system - return value as-is
+  if (fromUnitID === toUnitID && normalizedFromSystem === normalizedToSystem) {
+    return value;
+  }
+
   const fromDef = mergedUOMs[fromUnitID];
   const toDef = mergedUOMs[toUnitID];
 
@@ -138,5 +143,6 @@ export const convertUnit = (
   }
 
   // Convert via base unit: value * fromFactor / toFactor
-  return (value * fromFactor) / toFactor;
+  const result = (value * fromFactor) / toFactor;
+  return Math.round(result * 1e6) / 1e6;
 };
