diff --git a/README.md b/README.md
index a9ea7dd..59060c7 100644
--- a/README.md
+++ b/README.md
@@ -244,6 +244,7 @@ Notes:
 - Sender error counters are carried in the batch header and applied to all samples.
 - Receiver ACKs MeterBatch as soon as the batch is reassembled, before MQTT/web/UI work, to avoid missing the sender ACK window.
 - Receiver repeats ACKs (`ACK_REPEAT_COUNT`) spaced by `ACK_REPEAT_DELAY_MS` to cover sender RX latency.
+- Sender ACK RX window is derived from LoRa airtime (bounded min/max) and retried once if the first window misses.
 
 ## Device IDs
 - Derived from WiFi STA MAC.
diff --git a/src/main.cpp b/src/main.cpp
index 4b1e145..392235f 100644
--- a/src/main.cpp
+++ b/src/main.cpp
@@ -862,8 +862,24 @@ static void sender_loop() {
 
   if (g_batch_ack_pending) {
     LoraPacket ack_pkt = {};
+    const uint32_t ack_len = 5 + 6 + 2;
+    uint32_t ack_air_ms = lora_airtime_ms(ack_len);
+    uint32_t ack_window_ms = ack_air_ms + 300;
+    if (ack_window_ms < 1200) {
+      ack_window_ms = 1200;
+    }
+    if (ack_window_ms > 4000) {
+      ack_window_ms = 4000;
+    }
+    if (SERIAL_DEBUG_MODE) {
+      serial_debug_printf("ack: rx window=%lu airtime=%lu", static_cast<unsigned long>(ack_window_ms),
+                          static_cast<unsigned long>(ack_air_ms));
+    }
     uint32_t rx_start = millis();
-    bool got_ack = lora_receive_window(ack_pkt, 400);
+    bool got_ack = lora_receive_window(ack_pkt, ack_window_ms);
+    if (!got_ack) {
+      got_ack = lora_receive_window(ack_pkt, ack_window_ms / 2);
+    }
     uint32_t rx_elapsed = millis() - rx_start;
     if (SERIAL_DEBUG_MODE) {
       g_sender_rx_window_ms += rx_elapsed;