diff --git a/packages/server-admin-ui/src/store/slices/dataSlice.ts b/packages/server-admin-ui/src/store/slices/dataSlice.ts
index d0186eaea..7b4972cb2 100644
--- a/packages/server-admin-ui/src/store/slices/dataSlice.ts
+++ b/packages/server-admin-ui/src/store/slices/dataSlice.ts
@@ -95,7 +95,7 @@ export const createDataSlice: StateCreator<DataSlice, [], [], DataSlice> = (
 
       const newContextMeta = {
         ...contextMeta,
-        [path]: metaData as MetaData
+        [path]: { ...contextMeta[path], ...metaData } as MetaData
       }
 
       return {
diff --git a/packages/server-admin-ui/src/utils/unitConversion.ts b/packages/server-admin-ui/src/utils/unitConversion.ts
index 636e2686a..e5492e0b6 100644
--- a/packages/server-admin-ui/src/utils/unitConversion.ts
+++ b/packages/server-admin-ui/src/utils/unitConversion.ts
@@ -46,7 +46,8 @@ export function convertValue(
   siUnit: string,
   category: string,
   presetDetails: PresetDetails | null,
-  unitDefinitions: UnitDefinitions | null
+  unitDefinitions: UnitDefinitions | null,
+  displayUnits?: { targetUnit?: string; formula?: string; symbol?: string }
 ): ConvertedValue | null {
   if (typeof value !== 'number' || !category) {
     return null
@@ -55,6 +56,19 @@ export function convertValue(
   if (category === 'base' && siUnit) {
     return { value, unit: siUnit }
   }
+  // "custom" category uses explicitly stored formula/targetUnit
+  if (category === 'custom' && displayUnits?.formula) {
+    try {
+      const compiled = getCompiledFormula(displayUnits.formula)
+      const converted = compiled.evaluate({ value })
+      return {
+        value: converted,
+        unit: displayUnits.symbol || displayUnits.targetUnit || ''
+      }
+    } catch {
+      return null
+    }
+  }
   if (!presetDetails || !unitDefinitions) {
     return null
   }
diff --git a/packages/server-admin-ui/src/views/DataBrowser/DataRow.tsx b/packages/server-admin-ui/src/views/DataBrowser/DataRow.tsx
index 7085a97b1..12c462b32 100644
--- a/packages/server-admin-ui/src/views/DataBrowser/DataRow.tsx
+++ b/packages/server-admin-ui/src/views/DataBrowser/DataRow.tsx
@@ -130,6 +130,16 @@ function DataRow({
     category = findCategoryForPath(data.path, defaultCategories) ?? undefined
   }
 
+  const displayUnits =
+    (meta as Record<string, unknown> | null)?.displayUnits &&
+    typeof (meta as Record<string, unknown>).displayUnits === 'object'
+      ? ((meta as Record<string, unknown>).displayUnits as {
+          targetUnit?: string
+          formula?: string
+          symbol?: string
+        })
+      : undefined
+
   let convertedValue: number | null = null
   let convertedUnit: string | null = null
   if (category && typeof data.value === 'number') {
@@ -138,7 +148,8 @@ function DataRow({
       units,
       category,
       presetDetails,
-      unitDefinitions
+      unitDefinitions,
+      displayUnits
     )
     if (converted && converted.unit !== units) {
       convertedValue = converted.value
diff --git a/packages/server-admin-ui/src/views/DataBrowser/Meta.tsx b/packages/server-admin-ui/src/views/DataBrowser/Meta.tsx
index 7fe029988..41dbe124d 100644
--- a/packages/server-admin-ui/src/views/DataBrowser/Meta.tsx
+++ b/packages/server-admin-ui/src/views/DataBrowser/Meta.tsx
@@ -95,6 +95,7 @@ interface MetaFormRowProps {
   idPrefix: string
   categories?: string[]
   siUnit?: string
+  unitDefinitions?: UnitDefinitions | null
   description?: string
 }
 
@@ -203,6 +204,7 @@ const CATEGORY_BADGE_COLORS: Record<string, string> = {
 interface CategorySelectProps extends ValueRenderProps {
   categories?: string[]
   siUnit?: string
+  unitDefinitions?: UnitDefinitions | null
 }
 
 const CategorySelect: React.FC<CategorySelectProps> = ({
@@ -211,28 +213,65 @@ const CategorySelect: React.FC<CategorySelectProps> = ({
   setValue,
   inputId,
   categories,
-  siUnit
+  siUnit,
+  unitDefinitions
 }) => {
   const displayUnits = value as DisplayUnits | undefined
   const category = displayUnits?.category || ''
   const categoryList =
     categories !== undefined ? categories : DEFAULT_CATEGORIES
+  const conversions =
+    siUnit && unitDefinitions ? unitDefinitions[siUnit]?.conversions : undefined
   return (
-    <Form.Select
-      id={inputId}
-      disabled={disabled}
-      value={category}
-      size="sm"
-      onChange={(e) => setValue({ category: e.target.value })}
-    >
-      <option value="">-- No category --</option>
-      {siUnit && <option value="base">base ({siUnit})</option>}
-      {categoryList.map((cat) => (
-        <option key={cat} value={cat}>
-          {cat}
-        </option>
-      ))}
-    </Form.Select>
+    <>
+      <Form.Select
+        id={inputId}
+        disabled={disabled}
+        value={category}
+        size="sm"
+        onChange={(e) => setValue({ category: e.target.value })}
+      >
+        <option value="">-- No category --</option>
+        {siUnit && <option value="base">base ({siUnit})</option>}
+        {conversions && <option value="custom">custom unit</option>}
+        {categoryList.map((cat) => (
+          <option key={cat} value={cat}>
+            {cat}
+          </option>
+        ))}
+      </Form.Select>
+      {category === 'custom' && conversions && (
+        <Form.Select
+          disabled={disabled}
+          value={displayUnits?.targetUnit || ''}
+          size="sm"
+          style={{ marginTop: '4px' }}
+          onChange={(e) => {
+            const targetUnit = e.target.value
+            if (!targetUnit) {
+              setValue({ category: 'custom' })
+              return
+            }
+            const conv = conversions[targetUnit]
+            setValue({
+              category: 'custom',
+              targetUnit,
+              formula: conv?.formula,
+              inverseFormula: conv?.inverseFormula,
+              symbol: conv?.symbol || targetUnit
+            })
+          }}
+        >
+          <option value="">-- Select unit --</option>
+          {Object.entries(conversions).map(([unit, conv]) => (
+            <option key={unit} value={unit}>
+              {conv.symbol || unit}
+              {conv.longName ? ` - ${conv.longName}` : ''}
+            </option>
+          ))}
+        </Form.Select>
+      )}
+    </>
   )
 }
 
@@ -453,7 +492,7 @@ const DisplaySelect: React.FC<ValueRenderProps> = ({
 }
 
 const DisplayUnitsView: React.FC<CategorySelectProps> = (props) => {
-  const { disabled, categories, siUnit } = props
+  const { disabled, categories, siUnit, unitDefinitions } = props
   if (disabled) {
     const displayUnits = props.value as DisplayUnits | undefined
     if (!displayUnits || !displayUnits.category) {
@@ -480,7 +519,14 @@ const DisplayUnitsView: React.FC<CategorySelectProps> = (props) => {
       </div>
     )
   }
-  return <CategorySelect {...props} categories={categories} siUnit={siUnit} />
+  return (
+    <CategorySelect
+      {...props}
+      categories={categories}
+      siUnit={siUnit}
+      unitDefinitions={unitDefinitions}
+    />
+  )
 }
 
 const METAFIELDRENDERERS: Record<
@@ -510,9 +556,14 @@ const METAFIELDRENDERERS: Record<
           {...p}
           categories={props.categories}
           siUnit={props.siUnit}
+          unitDefinitions={props.unitDefinitions}
         />
       )}
-      description="Category for unit conversion"
+      description={
+        (props.value as DisplayUnits)?.category !== 'custom'
+          ? 'Target category for unit conversion'
+          : ''
+      }
     />
   ),
   zones: () => <></>,
@@ -599,7 +650,8 @@ const Meta: React.FC<MetaProps> = ({ meta, path, context }) => {
     siUnit,
     category,
     presetDetails,
-    unitDefinitions
+    unitDefinitions,
+    localMeta.displayUnits
   )
 
   const handleEdit = () => {
@@ -771,6 +823,7 @@ const Meta: React.FC<MetaProps> = ({ meta, path, context }) => {
                     disabled: !isEditing,
                     categories: filteredCategories,
                     siUnit,
+                    unitDefinitions,
                     setValue: (metaFieldValue) =>
                       setLocalMeta({ ...localMeta, [key]: metaFieldValue }),
                     setKey: (metaFieldKey) => {
diff --git a/src/interfaces/ws.ts b/src/interfaces/ws.ts
index 31b9eefdd..1fffc9b5f 100644
--- a/src/interfaces/ws.ts
+++ b/src/interfaces/ws.ts
@@ -41,7 +41,11 @@ import {
   buildFlushDeltas
 } from '../LatestValuesAccumulator'
 import { getExternalPort } from '../ports'
-import { resolveDisplayUnits, getDefaultCategory } from '../unitpreferences'
+import {
+  resolveDisplayUnits,
+  getDefaultCategory,
+  DisplayUnitsMetadata
+} from '../unitpreferences'
 import { Delta, hasValues } from '@signalk/server-api'
 
 const debug = createDebug('signalk-server:interfaces:ws')
@@ -900,15 +904,18 @@ function handleValuesMeta(
         break
       } else {
         this.spark.sentMetaData[partialContextPathKey] = true
-        let meta = getMetadata(partialContextPathKey) as Record<
+        const meta = getMetadata(partialContextPathKey) as Record<
           string,
           unknown
         > | null
         if (meta) {
           // Clone and enhance metadata with displayUnits formulas
-          meta = JSON.parse(JSON.stringify(meta))
-          let storedDisplayUnits = (meta as Record<string, unknown>)
-            .displayUnits as { category?: string } | undefined
+          const metaClone: Record<string, unknown> = JSON.parse(
+            JSON.stringify(meta)
+          )
+          let storedDisplayUnits = metaClone.displayUnits as
+            | Record<string, unknown>
+            | undefined
           if (!storedDisplayUnits?.category && path) {
             const defaultCategory = getDefaultCategory(path)
             if (defaultCategory) {
@@ -918,12 +925,19 @@ function handleValuesMeta(
           if (storedDisplayUnits?.category) {
             const username = this.spark.request.skPrincipal?.identifier
             const enhanced = resolveDisplayUnits(
-              { category: storedDisplayUnits.category },
-              (meta as Record<string, unknown>).units as string | undefined,
+              storedDisplayUnits as {
+                category: string
+                targetUnit?: string
+                formula?: string
+                inverseFormula?: string
+                symbol?: string
+                displayFormat?: string
+              },
+              metaClone.units as string | undefined,
               username
             )
             if (enhanced) {
-              ;(meta as Record<string, unknown>).displayUnits = enhanced
+              metaClone.displayUnits = enhanced
             }
           }
           this.spark.write({
@@ -934,7 +948,7 @@ function handleValuesMeta(
                 meta: [
                   {
                     path: path,
-                    value: meta
+                    value: metaClone
                   }
                 ]
               }
@@ -1157,12 +1171,13 @@ function handleRealtimeConnection(
           const fullPath = 'vessels.self.' + path
           const pathMeta =
             (getMetadata(fullPath) as Record<string, unknown>) || {}
-          const category =
-            (pathMeta.displayUnits as { category?: string } | undefined)
-              ?.category || getDefaultCategory(path)
+          const storedDU = pathMeta.displayUnits as
+            | DisplayUnitsMetadata
+            | undefined
+          const category = storedDU?.category || getDefaultCategory(path)
           if (category) {
             const displayUnits = resolveDisplayUnits(
-              { category },
+              { ...storedDU, category },
               pathMeta.units as string | undefined,
               username
             )
diff --git a/src/unitpreferences/resolver.ts b/src/unitpreferences/resolver.ts
index cf992d4a4..3a379c1e2 100644
--- a/src/unitpreferences/resolver.ts
+++ b/src/unitpreferences/resolver.ts
@@ -37,6 +37,41 @@ export function resolveDisplayUnits(
     }
   }
 
+  // "custom" category stores explicit conversion info
+  if (category === 'custom') {
+    if (!storedDisplayUnits.targetUnit) {
+      return null
+    }
+    // If formula is stored, use it directly
+    if (storedDisplayUnits.formula) {
+      return {
+        category: 'custom',
+        targetUnit: storedDisplayUnits.targetUnit,
+        formula: storedDisplayUnits.formula,
+        inverseFormula: storedDisplayUnits.inverseFormula || '',
+        symbol: storedDisplayUnits.symbol || storedDisplayUnits.targetUnit,
+        displayFormat: storedDisplayUnits.displayFormat
+      }
+    }
+    // Otherwise look up from definitions using pathSiUnit
+    if (pathSiUnit) {
+      const definitions = getMergedDefinitions()
+      const conversion =
+        definitions[pathSiUnit]?.conversions?.[storedDisplayUnits.targetUnit]
+      if (conversion) {
+        return {
+          category: 'custom',
+          targetUnit: storedDisplayUnits.targetUnit,
+          formula: conversion.formula,
+          inverseFormula: conversion.inverseFormula,
+          symbol: conversion.symbol || storedDisplayUnits.targetUnit,
+          displayFormat: storedDisplayUnits.displayFormat
+        }
+      }
+    }
+    return null
+  }
+
   const categoriesData = getCategories()
   const definitions = getMergedDefinitions()
   const preset = username ? getActivePresetForUser(username) : getActivePreset()
@@ -98,6 +133,11 @@ export function validateCategoryAssignment(
     return null
   }
 
+  // "custom" category is always valid - user picks an explicit target unit
+  if (category === 'custom') {
+    return null
+  }
+
   const categoriesData = getCategories()
   const preset = getActivePreset()
 
diff --git a/src/unitpreferences/types.ts b/src/unitpreferences/types.ts
index 08c763fa5..0a3130fc5 100644
--- a/src/unitpreferences/types.ts
+++ b/src/unitpreferences/types.ts
@@ -51,6 +51,9 @@ export interface UnitPreferencesConfig {
 export interface DisplayUnitsMetadata {
   category: string
   targetUnit?: string // Only if path override
+  formula?: string // Only if custom category
+  inverseFormula?: string // Only if custom category
+  symbol?: string // Only if custom category
   displayFormat?: string // Only if path override
 }