diff --git a/replayer/metrics.py b/replayer/metrics.py
index f48cbf2..aaf9f90 100644
--- a/replayer/metrics.py
+++ b/replayer/metrics.py
@@ -101,7 +101,11 @@ def _stats(values: list[float | None]) -> dict[str, float] | None:
 
 
 def _percentile(sorted_vals: list[float], pct: float) -> float:
-    if len(sorted_vals) == 1:
+    n = len(sorted_vals)
+    if n == 1:
         return sorted_vals[0]
-    idx = round((len(sorted_vals) - 1) * pct)
-    return sorted_vals[idx]
+    rank = pct * (n - 1)
+    lo = int(rank)
+    hi = min(lo + 1, n - 1)
+    frac = rank - lo
+    return sorted_vals[lo] * (1 - frac) + sorted_vals[hi] * frac