diff --git a/src/helpers/radiolib/RadioLibWrappers.cpp b/src/helpers/radiolib/RadioLibWrappers.cpp
index 18251e43..cc1dc001 100644
--- a/src/helpers/radiolib/RadioLibWrappers.cpp
+++ b/src/helpers/radiolib/RadioLibWrappers.cpp
@@ -139,6 +139,11 @@ int RadioLibWrapper::recvRaw(uint8_t* bytes, int sz) {
 }
 
 uint32_t RadioLibWrapper::getEstAirtimeFor(int len_bytes) {
+  uint8_t sf = getSpreadingFactor();
+  if (sf != _preamble_sf) {
+    _preamble_sf = sf;
+    _radio->setPreambleLength(preambleLengthForSF(sf)); // sync preamble before airtime estimate
+  }
   return _radio->getTimeOnAir(len_bytes) / 1000;
 }