diff --git a/StringFormatter/Culture.cs b/StringFormatter/Culture.cs
index f03c04c..b40b527 100644
--- a/StringFormatter/Culture.cs
+++ b/StringFormatter/Culture.cs
@@ -22,6 +22,7 @@ sealed class CachedCulture {
         public readonly string PercentNegativePattern;
         public readonly string PercentPositivePattern;
         public readonly string PercentSymbol;
+        public readonly string PerMilleSymbol;
 
         public readonly string NegativeSign;
         public readonly string PositiveSign;
@@ -89,6 +90,7 @@ public CachedCulture (CultureInfo culture) {
             PercentNegativePattern = NegativePercentFormats[info.PercentNegativePattern];
             PercentPositivePattern = PositivePercentFormats[info.PercentPositivePattern];
             PercentSymbol = info.PercentSymbol;
+            PerMilleSymbol = info.PerMilleSymbol;
             NegativeSign = info.NegativeSign;
             PositiveSign = info.PositiveSign;
             NaN = info.NaNSymbol;
diff --git a/StringFormatter/CustomNumeric.cs b/StringFormatter/CustomNumeric.cs
index e2e9012..812f63d 100644
--- a/StringFormatter/CustomNumeric.cs
+++ b/StringFormatter/CustomNumeric.cs
@@ -1,13 +1,328 @@
-using System;
-using System.Collections.Generic;
-using System.Linq;
-using System.Text;
-using System.Threading.Tasks;
-
 namespace System.Text.Formatting {
-    // this file contains the custom numeric formatting routines split out from the Numeric.cs file
-    partial class Numeric {
-        static void NumberToCustomFormatString (StringBuffer formatter, ref Number number, StringView specifier, CachedCulture culture) {
-        }
-    }
+	// this file contains the custom numeric formatting routines split out from the Numeric.cs file
+	unsafe partial class Numeric {
+		static void NumberToCustomFormatString(StringBuffer formatter, ref Number number, StringView specifier, CachedCulture culture) {
+			// Special: Handle special values
+			switch (number.Scale) {
+				case ScaleInf: formatter.Append(number.Sign == 0 ? culture.PositiveInfinity : culture.NegativeInfinity); return;
+				case ScaleNaN: formatter.Append(culture.NaN); return;
+			}
+			// Iteration 1: Split by semicolon
+			int specifierPositiveEnd = IndexOfSectionSeparator(specifier);
+			int specifierNegativeStart = 0, specifierNegativeEnd, specifierZeroStart = 0, specifierZeroEnd;
+			if (specifierPositiveEnd == -1) {
+				specifierPositiveEnd = specifierNegativeEnd = specifierZeroEnd = specifier.Length;
+			}
+			else {
+				specifierNegativeStart = specifierPositiveEnd + 1;
+				specifierNegativeEnd = IndexOfSectionSeparator(specifier, specifierNegativeStart);
+				if (specifierNegativeEnd == -1) {
+					specifierNegativeEnd = specifier.Length;
+					specifierZeroEnd = specifierPositiveEnd;
+				}
+				else {
+					specifierZeroStart = specifierNegativeEnd + 1;
+					specifierZeroEnd = specifier.Length;
+				}
+			}
+			// Special: Handle zero
+			if (IsZero(ref number)) {
+				FormatCustomFormatString(formatter, ref number, null, specifier, specifierZeroStart, specifierZeroEnd, culture);
+				return;
+			}
+			// Iteration 2: Divide and round number
+			int originalScale = number.Scale;
+			if (number.Sign == 0) ApplyDivisionAndPrecision(ref number, specifier, 0, specifierPositiveEnd);
+			else ApplyDivisionAndPrecision(ref number, specifier, specifierNegativeStart, specifierNegativeEnd);
+			// Iteration 3: Count; Iteration 4: Format
+			if (IsZero(ref number)) FormatCustomFormatString(formatter, ref number, null, specifier, specifierZeroStart, specifierZeroEnd, culture);
+			else if (number.Sign == 0) FormatCustomFormatString(formatter, ref number, originalScale, specifier, 0, specifierPositiveEnd, culture);
+			else {
+				if (specifierNegativeStart == 0) formatter.Append(culture.NegativeSign);
+				FormatCustomFormatString(formatter, ref number, originalScale, specifier, specifierNegativeStart, specifierNegativeEnd, culture);
+			}
+		}
+		static int IndexOfSectionSeparator(StringView specifier) {
+			return IndexOfSectionSeparator(specifier, 0);
+		}
+		static int IndexOfSectionSeparator(StringView specifier, int index) {
+			if (index < 0 || index > specifier.Length) throw new ArgumentOutOfRangeException("index");
+			char* ptr = specifier.Data;
+			for (; index < specifier.Length; index++) {
+				switch (ptr[index]) {
+					case ';': return index;
+					case '\\':
+						index++;
+						if (index >= specifier.Length) throw new FormatException();
+						break;
+					case '\'':
+						SkipLiteral(specifier, ref index, '\'');
+						break;
+					case '"':
+						SkipLiteral(specifier, ref index, '"');
+						break;
+				}
+			}
+			return -1;
+		}
+		static bool IsZero(ref Number number) {
+			char* ptr = number.Digits;
+			while (*ptr != '\0') if (*ptr++ != '0') return false;
+			return true;
+		}
+		static void ApplyDivisionAndPrecision(ref Number number, StringView specifier, int index, int end) {
+			int deltaScale = 0, scalingSpecifiers = 0;
+			int integralDigits = 0, decimalDigits = 0;
+			bool decimalFlag = false, exponentialFlag = false;
+			char* ptr = specifier.Data;
+			for (; index < end; index++) {
+				switch (ptr[index]) {
+					case '\\':
+						if (++index >= end) throw new FormatException();
+						break;
+					case '\'':
+						SkipLiteral(specifier, ref index, '\'');
+						break;
+					case '"':
+						SkipLiteral(specifier, ref index, '"');
+						break;
+					case '0':
+					case '#':
+						if (decimalFlag) decimalDigits++;
+						else integralDigits++;
+						scalingSpecifiers = 0;
+						break;
+					case '.': decimalFlag = true; deltaScale -= scalingSpecifiers * 3; break;
+					case ',': scalingSpecifiers++; break;
+					case '%': deltaScale += 2; break;
+					case '‰': deltaScale += 3; break;
+					case 'E':
+					case 'e':
+						if (++index >= end) goto exit;
+						char tc0 = ptr[index];
+						if (tc0 == '+' || tc0 == '-') {
+							if (++index >= end) goto exit;
+						}
+						if (ptr[index] != '0') break;
+						exponentialFlag = true;
+						for (index++; index < end && ptr[index] == '0'; index++) ;
+						index--;
+						break;
+				}
+			}
+		exit:
+			if (exponentialFlag) {
+				number.Scale = integralDigits;
+				RoundNumber(ref number, integralDigits + decimalDigits);
+			}
+			else {
+				number.Scale += deltaScale;
+				RoundNumber(ref number, number.Scale + decimalDigits);
+			}
+		}
+		static void FormatCustomFormatString(StringBuffer formatter, ref Number number, int? originalScale, StringView specifier, int index, int end, CachedCulture culture) {
+			int start = index;
+			int integralDigits = 0, integralZeros = 0,
+				decimalDigits  = 0, decimalZeros  = 0;
+			bool integralHasZeroFlag = false;
+			int exponentialZeros = 0;
+			bool decimalFlag = false;
+			bool commaFlag = false, groupFlag = false;
+			char* ptr = specifier.Data;
+			for (; index < end; index++) {
+				switch (ptr[index]) {
+					case '\\':
+						if (++index >= end) throw new FormatException();
+						break;
+					case '\'':
+						SkipLiteral(specifier, ref index, '\'');
+						break;
+					case '"':
+						SkipLiteral(specifier, ref index, '"');
+						break;
+					case '0':
+						if (decimalFlag) {
+							if (decimalZeros < decimalDigits)
+								decimalZeros = decimalDigits;
+							decimalDigits++;
+							decimalZeros++;
+						}
+						else {
+							integralDigits++;
+							integralZeros++;
+							integralHasZeroFlag = true;
+							if (commaFlag) groupFlag = true;
+						}
+						break;
+					case '#':
+						if (decimalFlag) {
+							decimalDigits++;
+						}
+						else {
+							integralDigits++;
+							if (integralHasZeroFlag) integralZeros++;
+							if (commaFlag) groupFlag = true;
+						}
+						break;
+					case '.': decimalFlag = true; commaFlag = false; break;
+					case ',': commaFlag = true; break;
+					case 'E':
+					case 'e':
+						if (++index >= end) goto exit;
+						char tc0 = ptr[index];
+						if (tc0 == '+' || tc0 == '-') {
+							if (++index >= end) goto exit;
+						}
+						if (ptr[index] != '0') break;
+						for (; index < end && ptr[index] == '0'; index++) exponentialZeros++;
+						index--;
+						break;
+				}
+			}
+		exit:
+			int currentDigitIndex = 0;
+			int groupIndex = 0, remainingDigitsInGroup = Math.Max(number.Scale, decimalZeros);
+			if (groupFlag) while (true) {
+				int groupSize = culture.NumberData.GroupSizes.ElementAtOrLast(groupIndex);
+				if (remainingDigitsInGroup <= groupSize) break;
+				remainingDigitsInGroup -= groupSize;
+				groupIndex++;
+			}
+			bool appendingDigitFlag = false, outOfPrecisionFlag = false;
+			decimalFlag = false;
+			for (index = start; index < end; index++) {
+				switch (ptr[index]) {
+					case '\\':
+						if (++index >= end) throw new FormatException();
+						formatter.Append(ptr[index]);
+						break;
+					case '\'':
+						formatter.AppendLiteral(specifier, ref index, '\'');
+						break;
+					case '"':
+						formatter.AppendLiteral(specifier, ref index, '"');
+						break;
+					case '0':
+					case '#':
+						if (!appendingDigitFlag) {
+							if (number.Scale > integralDigits) while (currentDigitIndex < number.Scale - integralDigits)
+								formatter.AppendIntegralDigit(ref number, ref currentDigitIndex, culture, groupFlag, ref outOfPrecisionFlag, ref groupIndex, ref remainingDigitsInGroup);
+							appendingDigitFlag = true;
+						}
+						if (decimalFlag) {
+							char digit = '0';
+							if (!outOfPrecisionFlag) {
+								if (currentDigitIndex >= number.Precision) outOfPrecisionFlag = true;
+								else {
+									digit = number.Digits[currentDigitIndex++];
+									if (digit == '\0') {
+										outOfPrecisionFlag = true;
+										digit = '0';
+									}
+								}
+								formatter.Append(digit);
+							}
+							else if (decimalZeros > 0)
+								formatter.Append('0');
+							--decimalDigits;
+							--decimalZeros;
+						}
+						else {
+							if (integralDigits <= number.Scale)
+								formatter.AppendIntegralDigit(ref number, ref currentDigitIndex, culture, groupFlag, ref outOfPrecisionFlag, ref groupIndex, ref remainingDigitsInGroup);
+							else if (integralDigits <= integralZeros)
+								formatter.AppendIntegralDigit('0', culture, groupFlag, ref groupIndex, ref remainingDigitsInGroup);
+							--integralDigits;
+						}
+						break;
+					case '.': formatter.Append(culture.NumberData.DecimalSeparator); decimalFlag = true; break;
+					case ',': break;
+					case '%': formatter.Append(culture.PercentSymbol); break;
+					case '‰': formatter.Append(culture.PerMilleSymbol); break;
+					case 'E':
+					case 'e':
+						char exponentialSymbol = ptr[index];
+						if (++index >= end) {
+							index--;
+							goto default;
+						}
+						char tc0 = ptr[index];
+						bool hasPlusFlag = false;
+						if (tc0 == '+' || tc0 == '-') {
+							if (++index >= end) {
+								index -= 2;
+								goto default;
+							}
+							if (tc0 == '+') hasPlusFlag = true;
+						}
+						if (ptr[index] != '0') {
+							index -= 2;
+							goto default;
+						}
+						for (index++; index < end && ptr[index] == '0'; index++) ;
+						index--;
+
+						int exp = originalScale == null ? 0 : originalScale.Value - number.Scale;
+						formatter.Append(exponentialSymbol);
+						if (exp < 0) {
+							formatter.Append(culture.NegativeSign);
+							exp = -exp;
+						}
+						else if (hasPlusFlag) {
+							formatter.Append(culture.PositiveSign);
+						}
+						Int32ToDecStr(formatter, exp, exponentialZeros, "");
+						break;
+					default:
+						formatter.Append(ptr[index]);
+						break;
+				}
+			}
+		}
+		static void AppendIntegralDigit(this StringBuffer self, ref Number number, ref int index, CachedCulture culture, bool groupFlag, ref bool outOfPrecisionFlag, ref int groupIndex, ref int remainingDigitsInGroup) {
+			char digit = '0';
+			if (!outOfPrecisionFlag) {
+				if (index >= number.Precision) outOfPrecisionFlag = true;
+				else {
+					digit = number.Digits[index];
+					if (digit == '\0') {
+						outOfPrecisionFlag = true;
+						digit = '0';
+					}
+				}
+			}
+			self.AppendIntegralDigit(digit, culture, groupFlag, ref groupIndex, ref remainingDigitsInGroup);
+			index++;
+		}
+		static void AppendIntegralDigit(this StringBuffer self, char digit, CachedCulture culture, bool groupFlag, ref int groupIndex, ref int remainingDigitsInGroup) {
+			self.Append(digit);
+			if (!groupFlag) return;
+			if (--remainingDigitsInGroup == 0) {
+				if (--groupIndex >= 0) {
+					remainingDigitsInGroup = culture.NumberData.GroupSizes.ElementAtOrLast(groupIndex);
+					self.Append(culture.NumberData.GroupSeparator);
+				}
+			}
+		}
+		static T ElementAtOrLast<T>(this T[] arr, int index) {
+			if (index >= arr.Length) index = arr.Length - 1;
+			return arr[index];
+		}
+		static void SkipLiteral(StringView specifier, ref int index, char delimiter) {
+			while (++index < specifier.Length) {
+				if (specifier.Data[index] == delimiter) return;
+			}
+			throw new FormatException();
+		}
+		static void AppendLiteral(this StringBuffer self, StringView specifier, ref int index, char delimiter) {
+			int start = index + 1;
+			while (++index < specifier.Length) {
+				if (specifier.Data[index] == delimiter) {
+					self.Append(specifier.Data + start, index - start);
+					return;
+				}
+			}
+			throw new FormatException();
+		}
+	}
 }