diff --git a/app/Http/Controllers/Api/CashflowAnalyticsController.php b/app/Http/Controllers/Api/CashflowAnalyticsController.php
index 95943cce..cdad16c4 100644
--- a/app/Http/Controllers/Api/CashflowAnalyticsController.php
+++ b/app/Http/Controllers/Api/CashflowAnalyticsController.php
@@ -218,15 +218,12 @@ class CashflowAnalyticsController extends Controller
 
     private function sumTransactions(Collection $transactions, string $userCurrency, CategoryType $type): int
     {
-        return $transactions
-            ->filter(function (Transaction $transaction) use ($type): bool {
-                if ($transaction->categoryType() === $type) {
-                    return true;
-                }
+        $onSide = $type === CategoryType::Income
+            ? fn (Transaction $transaction): bool => $transaction->isIncomeSide()
+            : fn (Transaction $transaction): bool => $transaction->isExpenseSide();
 
-                return $transaction->category_id === null
-                    && $this->matchesSign($transaction->amount, $type === CategoryType::Income ? '>' : '<');
-            })
+        return $transactions
+            ->filter($onSide)
             ->sum(fn (Transaction $transaction): int => $this->convertTransactionAmount($transaction, $userCurrency));
     }
 
diff --git a/app/Http/Controllers/Api/TransactionAnalysisController.php b/app/Http/Controllers/Api/TransactionAnalysisController.php
index 2c471161..ed325060 100644
--- a/app/Http/Controllers/Api/TransactionAnalysisController.php
+++ b/app/Http/Controllers/Api/TransactionAnalysisController.php
@@ -2,7 +2,6 @@
 
 namespace App\Http\Controllers\Api;
 
-use App\Enums\CategoryType;
 use App\Http\Controllers\Api\Concerns\ConvertsTransactionCurrency;
 use App\Http\Controllers\Controller;
 use App\Http\Requests\IndexTransactionRequest;
@@ -95,9 +94,9 @@ class TransactionAnalysisController extends Controller
         $expense = 0;
 
         foreach ($transactions as $transaction) {
-            if ($this->isIncomeSide($transaction)) {
+            if ($transaction->isIncomeSide()) {
                 $income += $this->convertTransactionAmount($transaction, $currency);
-            } elseif ($this->isExpenseSide($transaction)) {
+            } elseif ($transaction->isExpenseSide()) {
                 $expense += $this->convertTransactionAmount($transaction, $currency);
             }
         }
@@ -128,7 +127,7 @@ class TransactionAnalysisController extends Controller
     private function categoryBreakdown(Collection $transactions, string $currency, string $userId): Collection
     {
         $expenses = $transactions->filter(
-            fn (Transaction $transaction): bool => $this->isExpenseSide($transaction),
+            fn (Transaction $transaction): bool => $transaction->isExpenseSide(),
         );
 
         $grouped = $expenses
@@ -187,7 +186,7 @@ class TransactionAnalysisController extends Controller
         $totals = [];
 
         foreach ($transactions as $transaction) {
-            if (! $this->isExpenseSide($transaction)) {
+            if (! $transaction->isExpenseSide()) {
                 continue;
             }
 
@@ -215,7 +214,7 @@ class TransactionAnalysisController extends Controller
         $totals = [];
 
         foreach ($transactions as $transaction) {
-            if (! $this->isExpenseSide($transaction)) {
+            if (! $transaction->isExpenseSide()) {
                 continue;
             }
 
@@ -246,7 +245,7 @@ class TransactionAnalysisController extends Controller
         $totals = [];
 
         foreach ($transactions as $transaction) {
-            if (! $this->isExpenseSide($transaction)) {
+            if (! $transaction->isExpenseSide()) {
                 continue;
             }
 
@@ -279,7 +278,7 @@ class TransactionAnalysisController extends Controller
     private function largestExpenses(Collection $transactions, string $currency): array
     {
         return $transactions
-            ->filter(fn (Transaction $transaction): bool => $this->isExpenseSide($transaction) && $transaction->amount < 0)
+            ->filter(fn (Transaction $transaction): bool => $transaction->isExpenseSide() && $transaction->amount < 0)
             ->sortBy(fn (Transaction $transaction): int => $this->convertTransactionAmount($transaction, $currency))
             ->take(self::LARGEST_EXPENSES_LIMIT)
             ->map(fn (Transaction $transaction): array => [
@@ -332,9 +331,9 @@ class TransactionAnalysisController extends Controller
             $key = $transaction->transaction_date->format($keyFormat);
             $buckets[$key] ??= ['income' => 0, 'expense' => 0];
 
-            if ($this->isIncomeSide($transaction)) {
+            if ($transaction->isIncomeSide()) {
                 $buckets[$key]['income'] += $this->convertTransactionAmount($transaction, $currency);
-            } elseif ($this->isExpenseSide($transaction)) {
+            } elseif ($transaction->isExpenseSide()) {
                 $buckets[$key]['expense'] -= $this->convertTransactionAmount($transaction, $currency);
             }
         }
@@ -377,27 +376,4 @@ class TransactionAnalysisController extends Controller
 
         return (int) $dates->min()->diffInDays($dates->max()) + 1;
     }
-
-    /**
-     * Whether a transaction belongs to the expense side: anything booked to an
-     * expense category (a refund there nets back out), plus uncategorized
-     * outflows. Transfers, savings and investments are internal movements, so
-     * they sit on neither side — matching the cashflow screen.
-     */
-    private function isExpenseSide(Transaction $transaction): bool
-    {
-        return $transaction->categoryType() === CategoryType::Expense
-            || ($transaction->category_id === null && $transaction->amount < 0);
-    }
-
-    /**
-     * Whether a transaction belongs to the income side: anything booked to an
-     * income category (a reversal there nets back out), plus uncategorized
-     * inflows. Internal movements are excluded for the same reason as expenses.
-     */
-    private function isIncomeSide(Transaction $transaction): bool
-    {
-        return $transaction->categoryType() === CategoryType::Income
-            || ($transaction->category_id === null && $transaction->amount > 0);
-    }
 }
diff --git a/app/Models/Transaction.php b/app/Models/Transaction.php
index 2fea5d2a..dadf0757 100644
--- a/app/Models/Transaction.php
+++ b/app/Models/Transaction.php
@@ -133,6 +133,28 @@ class Transaction extends Model
         return is_string($type) ? CategoryType::tryFrom($type) : null;
     }
 
+    /**
+     * Whether this transaction sits on the income side of a cashflow split:
+     * booked to an income category (a reversal there nets back out) or an
+     * uncategorized inflow. Internal movements (transfer, savings, investment)
+     * belong to neither side.
+     */
+    public function isIncomeSide(): bool
+    {
+        return $this->categoryType() === CategoryType::Income
+            || ($this->category_id === null && $this->amount > 0);
+    }
+
+    /**
+     * Whether this transaction sits on the expense side: booked to an expense
+     * category (a refund there nets back out) or an uncategorized outflow.
+     */
+    public function isExpenseSide(): bool
+    {
+        return $this->categoryType() === CategoryType::Expense
+            || ($this->category_id === null && $this->amount < 0);
+    }
+
     /** @return BelongsTo<AutomationRule, $this> */
     public function categorizedByRule(): BelongsTo
     {
diff --git a/tests/Feature/TransactionSideClassificationTest.php b/tests/Feature/TransactionSideClassificationTest.php
new file mode 100644
index 00000000..c3043ca8
--- /dev/null
+++ b/tests/Feature/TransactionSideClassificationTest.php
@@ -0,0 +1,64 @@
+<?php
+
+use App\Enums\CategoryType;
+use App\Models\Account;
+use App\Models\Category;
+use App\Models\Transaction;
+use App\Models\User;
+
+beforeEach(function () {
+    $this->user = User::factory()->create();
+    $this->account = Account::factory()->create(['user_id' => $this->user->id]);
+});
+
+function makeSideTransaction(User $user, Account $account, ?Category $category, int $amount): Transaction
+{
+    return Transaction::factory()->create([
+        'user_id' => $user->id,
+        'account_id' => $account->id,
+        'category_id' => $category?->id,
+        'amount' => $amount,
+    ])->load('category');
+}
+
+it('classifies an income-category transaction as income side regardless of sign', function () {
+    $income = Category::factory()->create(['user_id' => $this->user->id, 'type' => CategoryType::Income]);
+
+    $positive = makeSideTransaction($this->user, $this->account, $income, 10000);
+    $reversal = makeSideTransaction($this->user, $this->account, $income, -2000);
+
+    expect($positive->isIncomeSide())->toBeTrue()
+        ->and($positive->isExpenseSide())->toBeFalse()
+        ->and($reversal->isIncomeSide())->toBeTrue();
+});
+
+it('classifies an expense-category transaction as expense side regardless of sign', function () {
+    $expense = Category::factory()->create(['user_id' => $this->user->id, 'type' => CategoryType::Expense]);
+
+    $spend = makeSideTransaction($this->user, $this->account, $expense, -5000);
+    $refund = makeSideTransaction($this->user, $this->account, $expense, 1500);
+
+    expect($spend->isExpenseSide())->toBeTrue()
+        ->and($spend->isIncomeSide())->toBeFalse()
+        ->and($refund->isExpenseSide())->toBeTrue();
+});
+
+it('classifies uncategorized inflows as income and outflows as expense', function () {
+    $inflow = makeSideTransaction($this->user, $this->account, null, 7000);
+    $outflow = makeSideTransaction($this->user, $this->account, null, -3000);
+
+    expect($inflow->isIncomeSide())->toBeTrue()
+        ->and($inflow->isExpenseSide())->toBeFalse()
+        ->and($outflow->isExpenseSide())->toBeTrue()
+        ->and($outflow->isIncomeSide())->toBeFalse();
+});
+
+it('treats transfer, savings and investment categories as neither side', function () {
+    foreach ([CategoryType::Transfer, CategoryType::Savings, CategoryType::Investment] as $type) {
+        $category = Category::factory()->create(['user_id' => $this->user->id, 'type' => $type]);
+        $transaction = makeSideTransaction($this->user, $this->account, $category, -5000);
+
+        expect($transaction->isIncomeSide())->toBeFalse()
+            ->and($transaction->isExpenseSide())->toBeFalse();
+    }
+});