diff --git a/CHANGELOG.md b/CHANGELOG.md
index 264b0bcb9ea6..35320ebafbd7 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -7,7 +7,9 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
-- Nothing yet!
+### Fixed
+
+- Don’t break `sibling-*()` functions when used inside `calc(…)` ([#19335](https://github.com/tailwindlabs/tailwindcss/pull/19335))
 
 ## [3.4.18] - 2024-10-01
 
diff --git a/src/util/dataTypes.js b/src/util/dataTypes.js
index 9447732938cd..e3210e9b98fd 100644
--- a/src/util/dataTypes.js
+++ b/src/util/dataTypes.js
@@ -1,4 +1,5 @@
 import { parseColor } from './color'
+import { addWhitespaceAroundMathOperators } from './math-operators'
 import { parseBoxShadowValue } from './parseBoxShadowValue'
 import { splitAtTopLevelOnly } from './splitAtTopLevelOnly'
 
@@ -76,7 +77,7 @@ export function normalize(value, context = null, isRoot = true) {
     value = value.trim()
   }
 
-  value = normalizeMathOperatorSpacing(value)
+  value = addWhitespaceAroundMathOperators(value)
 
   return value
 }
@@ -109,122 +110,6 @@ export function normalizeAttributeSelectors(value) {
   return value
 }
 
-/**
- * Add spaces around operators inside math functions
- * like calc() that do not follow an operator, '(', or `,`.
- *
- * @param {string} value
- * @returns {string}
- */
-function normalizeMathOperatorSpacing(value) {
-  let preventFormattingInFunctions = ['theme']
-  let preventFormattingKeywords = [
-    'min-content',
-    'max-content',
-    'fit-content',
-
-    // Env
-    'safe-area-inset-top',
-    'safe-area-inset-right',
-    'safe-area-inset-bottom',
-    'safe-area-inset-left',
-
-    'titlebar-area-x',
-    'titlebar-area-y',
-    'titlebar-area-width',
-    'titlebar-area-height',
-
-    'keyboard-inset-top',
-    'keyboard-inset-right',
-    'keyboard-inset-bottom',
-    'keyboard-inset-left',
-    'keyboard-inset-width',
-    'keyboard-inset-height',
-
-    'radial-gradient',
-    'linear-gradient',
-    'conic-gradient',
-    'repeating-radial-gradient',
-    'repeating-linear-gradient',
-    'repeating-conic-gradient',
-
-    'anchor-size',
-  ]
-
-  return value.replace(/(calc|min|max|clamp)\(.+\)/g, (match) => {
-    let result = ''
-
-    function lastChar() {
-      let char = result.trimEnd()
-      return char[char.length - 1]
-    }
-
-    for (let i = 0; i < match.length; i++) {
-      function peek(word) {
-        return word.split('').every((char, j) => match[i + j] === char)
-      }
-
-      function consumeUntil(chars) {
-        let minIndex = Infinity
-        for (let char of chars) {
-          let index = match.indexOf(char, i)
-          if (index !== -1 && index < minIndex) {
-            minIndex = index
-          }
-        }
-
-        let result = match.slice(i, minIndex)
-        i += result.length - 1
-        return result
-      }
-
-      let char = match[i]
-
-      // Handle `var(--variable)`
-      if (peek('var')) {
-        // When we consume until `)`, then we are dealing with this scenario:
-        //   `var(--example)`
-        //
-        // When we consume until `,`, then we are dealing with this scenario:
-        //   `var(--example, 1rem)`
-        //
-        //   In this case we do want to "format", the default value as well
-        result += consumeUntil([')', ','])
-      }
-
-      // Skip formatting of known keywords
-      else if (preventFormattingKeywords.some((keyword) => peek(keyword))) {
-        let keyword = preventFormattingKeywords.find((keyword) => peek(keyword))
-        result += keyword
-        i += keyword.length - 1
-      }
-
-      // Skip formatting inside known functions
-      else if (preventFormattingInFunctions.some((fn) => peek(fn))) {
-        result += consumeUntil([')'])
-      }
-
-      // Don't break CSS grid track names
-      else if (peek('[')) {
-        result += consumeUntil([']'])
-      }
-
-      // Handle operators
-      else if (
-        ['+', '-', '*', '/'].includes(char) &&
-        !['(', '+', '-', '*', '/', ','].includes(lastChar())
-      ) {
-        result += ` ${char} `
-      } else {
-        result += char
-      }
-    }
-
-    // Simplify multiple spaces
-    return result.replace(/\s+/g, ' ')
-  })
-}
-
 export function url(value) {
   return value.startsWith('url(')
 }
diff --git a/src/util/math-operators.ts b/src/util/math-operators.ts
new file mode 100644
index 000000000000..d31cc3c526c9
--- /dev/null
+++ b/src/util/math-operators.ts
@@ -0,0 +1,205 @@
+const LOWER_A = 0x61
+const LOWER_Z = 0x7a
+const UPPER_A = 0x41
+const UPPER_Z = 0x5a
+const LOWER_E = 0x65
+const UPPER_E = 0x45
+const ZERO = 0x30
+const NINE = 0x39
+const ADD = 0x2b
+const SUB = 0x2d
+const MUL = 0x2a
+const DIV = 0x2f
+const OPEN_PAREN = 0x28
+const CLOSE_PAREN = 0x29
+const COMMA = 0x2c
+const SPACE = 0x20
+const PERCENT = 0x25
+
+const MATH_FUNCTIONS = [
+  'calc',
+  'min',
+  'max',
+  'clamp',
+  'mod',
+  'rem',
+  'sin',
+  'cos',
+  'tan',
+  'asin',
+  'acos',
+  'atan',
+  'atan2',
+  'pow',
+  'sqrt',
+  'hypot',
+  'log',
+  'exp',
+  'round',
+]
+
+export function hasMathFn(input: string) {
+  return input.indexOf('(') !== -1 && MATH_FUNCTIONS.some((fn) => input.includes(`${fn}(`))
+}
+
+export function addWhitespaceAroundMathOperators(input: string) {
+  // Bail early if there are no math functions in the input
+  if (!MATH_FUNCTIONS.some((fn) => input.includes(fn))) {
+    return input
+  }
+
+  let result = ''
+  let formattable: boolean[] = []
+
+  let valuePos = null
+  let lastValuePos = null
+
+  for (let i = 0; i < input.length; i++) {
+    let char = input.charCodeAt(i)
+
+    // Track if we see a number followed by a unit, then we know for sure that
+    // this is not a function call.
+    if (char >= ZERO && char <= NINE) {
+      valuePos = i
+    }
+
+    // If we saw a number before, and we see normal a-z character, then we
+    // assume this is a value such as `123px`
+    else if (
+      valuePos !== null &&
+      (char === PERCENT ||
+        (char >= LOWER_A && char <= LOWER_Z) ||
+        (char >= UPPER_A && char <= UPPER_Z))
+    ) {
+      valuePos = i
+    }
+
+    // Once we see something else, we reset the value position
+    else {
+      lastValuePos = valuePos
+      valuePos = null
+    }
+
+    // Determine if we're inside a math function
+    if (char === OPEN_PAREN) {
+      result += input[i]
+
+      // Scan backwards to determine the function name. This assumes math
+      // functions are named with lowercase alphanumeric characters.
+      let start = i
+
+      for (let j = i - 1; j >= 0; j--) {
+        let inner = input.charCodeAt(j)
+
+        if (inner >= ZERO && inner <= NINE) {
+          start = j // 0-9
+        } else if (inner >= LOWER_A && inner <= LOWER_Z) {
+          start = j // a-z
+        } else {
+          break
+        }
+      }
+
+      let fn = input.slice(start, i)
+
+      // This is a known math function so start formatting
+      if (MATH_FUNCTIONS.includes(fn)) {
+        formattable.unshift(true)
+        continue
+      }
+
+      // We've encountered nested parens inside a math function, record that and
+      // keep formatting until we've closed all parens.
+      else if (formattable[0] && fn === '') {
+        formattable.unshift(true)
+        continue
+      }
+
+      // This is not a known math function so don't format it
+      formattable.unshift(false)
+      continue
+    }
+
+    // We've exited the function so format according to the parent function's
+    // type.
+    else if (char === CLOSE_PAREN) {
+      result += input[i]
+      formattable.shift()
+    }
+
+    // Add spaces after commas in math functions
+    else if (char === COMMA && formattable[0]) {
+      result += `, `
+      continue
+    }
+
+    // Skip over consecutive whitespace
+    else if (char === SPACE && formattable[0] && result.charCodeAt(result.length - 1) === SPACE) {
+      continue
+    }
+
+    // Add whitespace around operators inside math functions
+    else if ((char === ADD || char === MUL || char === DIV || char === SUB) && formattable[0]) {
+      let trimmed = result.trimEnd()
+      let prev = trimmed.charCodeAt(trimmed.length - 1)
+      let prevPrev = trimmed.charCodeAt(trimmed.length - 2)
+      let next = input.charCodeAt(i + 1)
+
+      // Do not add spaces for scientific notation, e.g.: `-3.4e-2`
+      if ((prev === LOWER_E || prev === UPPER_E) && prevPrev >= ZERO && prevPrev <= NINE) {
+        result += input[i]
+        continue
+      }
+
+      // If we're preceded by an operator don't add spaces
+      else if (prev === ADD || prev === MUL || prev === DIV || prev === SUB) {
+        result += input[i]
+        continue
+      }
+
+      // If we're at the beginning of an argument don't add spaces
+      else if (prev === OPEN_PAREN || prev === COMMA) {
+        result += input[i]
+        continue
+      }
+
+      // Add spaces only after the operator if we already have spaces before it
+      else if (input.charCodeAt(i - 1) === SPACE) {
+        result += `${input[i]} `
+      }
+
+      // Add spaces around the operator, if...
+      else if (
+        // Previous is a digit
+        (prev >= ZERO && prev <= NINE) ||
+        // Next is a digit
+        (next >= ZERO && next <= NINE) ||
+        // Previous is end of a function call (or parenthesized expression)
+        prev === CLOSE_PAREN ||
+        // Next is start of a parenthesized expression
+        next === OPEN_PAREN ||
+        // Next is an operator
+        next === ADD ||
+        next === MUL ||
+        next === DIV ||
+        next === SUB ||
+        // Previous position was a value (+ unit)
+        (lastValuePos !== null && lastValuePos === i - 1)
+      ) {
+        result += ` ${input[i]} `
+      }
+
+      // Everything else
+      else {
+        result += input[i]
+      }
+    }
+
+    // Handle all other characters
+    else {
+      result += input[i]
+    }
+  }
+
+  return result
+}
diff --git a/tests/normalize-data-types.test.js b/tests/normalize-data-types.test.js
index 1c302ec28e68..8135a8ced477 100644
--- a/tests/normalize-data-types.test.js
+++ b/tests/normalize-data-types.test.js
@@ -37,7 +37,7 @@ let table = [
   ],
   ['min(1+2)', 'min(1 + 2)'],
   ['max(1+2)', 'max(1 + 2)'],
-  ['clamp(1+2,1+3,1+4)', 'clamp(1 + 2,1 + 3,1 + 4)'],
+  ['clamp(1+2,1+3,1+4)', 'clamp(1 + 2, 1 + 3, 1 + 4)'],
   ['var(--heading-h1-font-size)', 'var(--heading-h1-font-size)'],
   ['var(--my-var-with-more-than-3-words)', 'var(--my-var-with-more-than-3-words)'],
   ['var(--width, calc(100%+1rem))', 'var(--width, calc(100% + 1rem))'],
@@ -69,7 +69,7 @@ let table = [
   ['calc(theme(spacing.foo-bar))', 'calc(theme(spacing.foo-bar))'],
 
   // A negative number immediately after a `,` should not have spaces inserted
-  ['clamp(-3px+4px,-3px+4px,-3px+4px)', 'clamp(-3px + 4px,-3px + 4px,-3px + 4px)'],
+  ['clamp(-3px+4px,-3px+4px,-3px+4px)', 'clamp(-3px + 4px, -3px + 4px, -3px + 4px)'],
 
   // Prevent formatting inside `var()` functions
   ['calc(var(--foo-bar-bar)*2)', 'calc(var(--foo-bar-bar) * 2)'],
@@ -98,6 +98,10 @@ let table = [
 
   // Prevent formatting functions that are not math functions
   ['w-[calc(anchor-size(width)+8px)]', 'w-[calc(anchor-size(width) + 8px)]'],
+  ['w-[calc(sibling-index()*1%)]', 'w-[calc(sibling-index() * 1%)]'],
+  ['w-[calc(sibling-count()*1%)]', 'w-[calc(sibling-count() * 1%)]'],
+  ['w-[calc(--custom()*1%)]', 'w-[calc(--custom() * 1%)]'],
+  ['w-[calc(--custom-fn()*1%)]', 'w-[calc(--custom-fn() * 1%)]'],
 
   // Misc
   ['color(0_0_0/1.0)', 'color(0 0 0/1.0)'],