diff --git a/public/playlist.js b/public/playlist.js
index 444530e..b2338af 100644
--- a/public/playlist.js
+++ b/public/playlist.js
@@ -666,7 +666,7 @@
   var plTimeReadout = document.getElementById('plTimeReadout')
   var plTrimDuration = document.getElementById('plTrimDuration')
   var plStartSec = document.getElementById('plStartSec')
-  var plEndSec = document.getElementById('plEndSec')
+  var plLengthSec = document.getElementById('plLengthSec')
 
   var MIN_SEL = 0.05
   var trimPlayer = null
@@ -698,7 +698,8 @@
     plTrimHandleEnd.style.left = endPct + '%'
     plTrimDuration.textContent = '선택: ' + (end - tctx.start).toFixed(1) + '초'
     plStartSec.value = (tctx.start || 0).toFixed(2)
-    plEndSec.value = tctx.end == null ? '' : tctx.end.toFixed(2)
+    // 끝 대신 재생 길이(초)를 보여준다 (끝까지면 영상 끝 - 시작).
+    plLengthSec.value = Math.max(0, end - tctx.start).toFixed(2)
   }
   function renderPlayhead() {
     if (!tctx) return
@@ -868,15 +869,13 @@
     tctx.start = tctx.duration ? clampNum(v, 0, tEnd() - MIN_SEL) : v
     renderTrimBar()
   })
-  plEndSec.addEventListener('change', function () {
+  plLengthSec.addEventListener('change', function () {
     if (!tctx) return
-    if (plEndSec.value === '') {
-      tctx.end = null
-    } else {
-      var v = Number(plEndSec.value)
-      if (!isFinite(v) || v <= tctx.start) { renderTrimBar(); return }
-      tctx.end = tctx.duration ? clampNum(v, tctx.start + MIN_SEL, tctx.duration) : v
-    }
+    // 길이(초) 입력 → 끝 = 시작 + 길이.
+    var v = Number(plLengthSec.value)
+    if (!isFinite(v) || v <= 0) { renderTrimBar(); return }
+    var end = tctx.start + v
+    tctx.end = tctx.duration ? clampNum(end, tctx.start + MIN_SEL, tctx.duration) : end
     renderTrimBar()
   })
 
diff --git a/views/op/playlist.ejs b/views/op/playlist.ejs
index 65178cf..aabb510 100644
--- a/views/op/playlist.ejs
+++ b/views/op/playlist.ejs
@@ -99,7 +99,7 @@
         <div class="trimNumericRow">
           <label>시작(초) <input type="number" id="plStartSec" step="0.1" min="0" value="0" /></label>
           <span class="trimDuration" id="plTrimDuration">선택: 0.0초</span>
-          <label>끝(초, 비우면 끝까지) <input type="number" id="plEndSec" step="0.1" min="0" value="" /></label>
+          <label>길이(초) <input type="number" id="plLengthSec" step="0.1" min="0.1" value="" /></label>
         </div>
         <p class="muted">재생바의 파란 핸들을 끌어 구간을 정합니다. 적용하면 항목의 시작/길이가 갱신되고, 다운로드 완료 후 이 구간으로 잘립니다.</p>
       </div>