diff --git a/src/app/api/analytics/fees/route.ts b/src/app/api/analytics/fees/route.ts
index 0054c5c..57657ad 100644
--- a/src/app/api/analytics/fees/route.ts
+++ b/src/app/api/analytics/fees/route.ts
@@ -1,7 +1,7 @@
 import { NextRequest, NextResponse } from "next/server";
 import { getCurrentUser } from "@/lib/auth";
 import { queryRaw } from "@/lib/db";
-import { OWNER_SCOPE, STATEMENTS_JOIN } from "@/lib/analytics-sql";
+import { OWNER_SCOPE, STATEMENTS_JOIN, mySplitOf } from "@/lib/analytics-sql";
 
 export async function GET(req: NextRequest) {
   const user = await getCurrentUser(req);
@@ -41,11 +41,7 @@ export async function GET(req: NextRequest) {
       t.description,
       t.merchant_name,
       t.transaction_type,
-      CASE
-        WHEN ts.share_percent IS NOT NULL THEN COALESCE(t.amount_aud, t.amount) * ts.share_percent / 100
-        WHEN o.my_share_percent IS NOT NULL THEN COALESCE(t.amount_aud, t.amount) * o.my_share_percent / 100
-        ELSE COALESCE(t.amount_aud, t.amount)
-      END::numeric(12,2) AS my_amount,
+      ${mySplitOf(`COALESCE(t.amount_aud, t.amount)`)}::numeric(12,2) AS my_amount,
       COALESCE(s.bank_name, 'Manual') AS bank_name
     FROM transactions t
     LEFT JOIN transaction_overrides o ON o.transaction_id = t.id
diff --git a/src/app/api/analytics/merchants/[merchant]/route.ts b/src/app/api/analytics/merchants/[merchant]/route.ts
index 2361bd3..9abac5d 100644
--- a/src/app/api/analytics/merchants/[merchant]/route.ts
+++ b/src/app/api/analytics/merchants/[merchant]/route.ts
@@ -1,7 +1,9 @@
 import { NextRequest, NextResponse } from "next/server";
 import { getCurrentUser } from "@/lib/auth";
 import { queryRaw } from "@/lib/db";
-import { OWNER_SCOPE, STATEMENTS_JOIN, EFFECTIVE_CATEGORY } from "@/lib/analytics-sql";
+import { OWNER_SCOPE, STATEMENTS_JOIN, EFFECTIVE_CATEGORY, mySplitOf } from "@/lib/analytics-sql";
+
+const MY_AMOUNT = mySplitOf(`COALESCE(t.amount_aud, t.amount)`);
 
 export async function GET(
   req: NextRequest,
@@ -32,10 +34,8 @@ export async function GET(
       t.amount,
       t.amount_aud,
       CASE
-        WHEN t.transaction_type IN ('refund', 'credit') THEN
-          -(CASE WHEN ts.share_percent IS NOT NULL THEN COALESCE(t.amount_aud, t.amount) * ts.share_percent / 100 WHEN o.my_share_percent IS NOT NULL THEN COALESCE(t.amount_aud, t.amount) * o.my_share_percent / 100 ELSE COALESCE(t.amount_aud, t.amount) END)
-        ELSE
-          (CASE WHEN ts.share_percent IS NOT NULL THEN COALESCE(t.amount_aud, t.amount) * ts.share_percent / 100 WHEN o.my_share_percent IS NOT NULL THEN COALESCE(t.amount_aud, t.amount) * o.my_share_percent / 100 ELSE COALESCE(t.amount_aud, t.amount) END)
+        WHEN t.transaction_type IN ('refund', 'credit') THEN -${MY_AMOUNT}
+        ELSE ${MY_AMOUNT}
       END::numeric(10,2) as my_amount,
       t.transaction_type,
       ${EFFECTIVE_CATEGORY} as category,
diff --git a/src/app/api/analytics/merchants/route.ts b/src/app/api/analytics/merchants/route.ts
index 23cdc41..dffc6cf 100644
--- a/src/app/api/analytics/merchants/route.ts
+++ b/src/app/api/analytics/merchants/route.ts
@@ -1,10 +1,10 @@
 import { NextRequest, NextResponse } from "next/server";
 import { getCurrentUser } from "@/lib/auth";
 import { queryRaw } from "@/lib/db";
-import { OWNER_SCOPE, STATEMENTS_JOIN, EXCLUDE_NON_SPEND, EFFECTIVE_CATEGORY } from "@/lib/analytics-sql";
+import { OWNER_SCOPE, STATEMENTS_JOIN, EXCLUDE_NON_SPEND, EFFECTIVE_CATEGORY, mySplitOf } from "@/lib/analytics-sql";
 
 // Split-adjusted amount helper (positive for spend, negative for refunds)
-const MY_AMOUNT = `CASE WHEN ts.share_percent IS NOT NULL THEN COALESCE(t.amount_aud, t.amount) * ts.share_percent / 100 WHEN o.my_share_percent IS NOT NULL THEN COALESCE(t.amount_aud, t.amount) * o.my_share_percent / 100 ELSE COALESCE(t.amount_aud, t.amount) END`;
+const MY_AMOUNT = mySplitOf(`COALESCE(t.amount_aud, t.amount)`);
 const SPEND_EXPR = `
   CASE
     WHEN t.transaction_type IN ('refund', 'credit') THEN -(${MY_AMOUNT})
diff --git a/src/app/api/analytics/monthly/route.ts b/src/app/api/analytics/monthly/route.ts
index 9cd052a..48a1f3b 100644
--- a/src/app/api/analytics/monthly/route.ts
+++ b/src/app/api/analytics/monthly/route.ts
@@ -8,6 +8,7 @@ import {
   EXCLUDE_NON_SPEND,
   SPEND_ROWS,
   SPEND_BASE,
+  mySplitOf,
 } from "@/lib/analytics-sql";
 
 export async function GET(req: NextRequest) {
@@ -34,13 +35,7 @@ export async function GET(req: NextRequest) {
     `SELECT
       TO_CHAR(DATE_TRUNC('month', t.transaction_date::date), 'YYYY-MM') as month,
       ${EFFECTIVE_CATEGORY} as category,
-      SUM(
-        CASE
-          WHEN ts.share_percent IS NOT NULL THEN (${SPEND_BASE}) * ts.share_percent / 100
-          WHEN o.my_share_percent IS NOT NULL THEN (${SPEND_BASE}) * o.my_share_percent / 100
-          ELSE (${SPEND_BASE})
-        END
-      )::numeric(12,2) as total_spent,
+      SUM(${mySplitOf(SPEND_BASE)})::numeric(12,2) as total_spent,
       COUNT(*)::int as transaction_count
     FROM transactions t
     LEFT JOIN transaction_overrides o ON o.transaction_id = t.id
diff --git a/src/app/api/analytics/subscriptions/route.ts b/src/app/api/analytics/subscriptions/route.ts
index d5b0c20..335430f 100644
--- a/src/app/api/analytics/subscriptions/route.ts
+++ b/src/app/api/analytics/subscriptions/route.ts
@@ -1,7 +1,7 @@
 import { NextRequest, NextResponse } from "next/server";
 import { getCurrentUser } from "@/lib/auth";
 import { queryRaw } from "@/lib/db";
-import { OWNER_SCOPE, STATEMENTS_JOIN, EXCLUDE_NON_SPEND, EFFECTIVE_CATEGORY } from "@/lib/analytics-sql";
+import { OWNER_SCOPE, STATEMENTS_JOIN, EXCLUDE_NON_SPEND, EFFECTIVE_CATEGORY, mySplitOf } from "@/lib/analytics-sql";
 
 export async function GET(req: NextRequest) {
   const user = await getCurrentUser(req);
@@ -23,11 +23,7 @@ export async function GET(req: NextRequest) {
         COALESCE(o.merchant_normalized, t.merchant_normalized, t.merchant_name) AS merchant,
         ${EFFECTIVE_CATEGORY} AS category,
         t.transaction_date,
-        CASE
-          WHEN ts.share_percent IS NOT NULL THEN COALESCE(t.amount_aud, t.amount) * ts.share_percent / 100
-          WHEN o.my_share_percent IS NOT NULL THEN COALESCE(t.amount_aud, t.amount) * o.my_share_percent / 100
-          ELSE COALESCE(t.amount_aud, t.amount)
-        END AS my_amount
+        ${mySplitOf(`COALESCE(t.amount_aud, t.amount)`)} AS my_amount
       FROM transactions t
       LEFT JOIN transaction_overrides o ON o.transaction_id = t.id
       LEFT JOIN transaction_splits ts ON ts.transaction_id = t.id AND ts.participant_id = $1
diff --git a/src/lib/analytics-sql.ts b/src/lib/analytics-sql.ts
index 6c8e3cc..fbe86e3 100644
--- a/src/lib/analytics-sql.ts
+++ b/src/lib/analytics-sql.ts
@@ -42,6 +42,35 @@ export const SPEND_BASE = `CASE WHEN t.interest_amount IS NOT NULL THEN t.intere
 /** Effective category, honouring overrides. Never NULL. */
 export const EFFECTIVE_CATEGORY = `COALESCE(o.category_override, t.category, 'other')`;
 
+/**
+ * My percentage share of a transaction, 0-100.
+ *
+ * Resolution order:
+ *   1. An explicit `transaction_splits` row for me.
+ *   2. The `my_share_percent` override.
+ *   3. Whatever is left after everyone else's shares.
+ *
+ * Step 3 is the one that matters. Assuming 100% when no split row exists for me
+ * is wrong whenever a transaction is allocated entirely to someone else — I paid,
+ * they owe all of it, and there is no row for me to find. Those rows would
+ * otherwise land in my spend at full value.
+ *
+ * Requires `transaction_splits ts` joined on `ts.participant_id = <participant>`
+ * and `transaction_overrides o` joined on the transaction.
+ */
+export const myShare = (participant = "$1") => `COALESCE(
+    ts.share_percent,
+    o.my_share_percent,
+    100 - COALESCE((
+      SELECT SUM(x.share_percent) FROM transaction_splits x
+      WHERE x.transaction_id = t.id AND x.participant_id <> ${participant}
+    ), 0)
+  )`;
+
+/** `base` scaled to my share. Use for every per-user spend total. */
+export const mySplitOf = (base: string, participant = "$1") =>
+  `((${base}) * ${myShare(participant)} / 100)`;
+
 /**
  * Predicate excluding money-movement categories.
  * The COALESCE matters: a bare `category NOT IN (...)` evaluates to NULL for