diff --git a/libavcodec/acelp_pitch_delay.c b/libavcodec/acelp_pitch_delay.c
index ac929c4689..4c809d7eeb 100644
--- a/libavcodec/acelp_pitch_delay.c
+++ b/libavcodec/acelp_pitch_delay.c
@@ -119,3 +119,24 @@ int16_t ff_acelp_decode_gain_code(
     return mr_energy >> 12;
 #endif
 }
+
+float ff_amr_set_fixed_gain(float fixed_gain_factor, float fixed_mean_energy,
+                            float *prediction_error, float energy_mean,
+                            const float *pred_table)
+{
+    // Equations 66-69:
+    // ^g_c = ^gamma_gc * 100.05 (predicted dB + mean dB - dB of fixed vector)
+    // Note 10^(0.05 * -10log(average x2)) = 1/sqrt((average x2)).
+    float val = fixed_gain_factor *
+        exp2f(log2f(10.0) * 0.05 *
+              (ff_dot_productf(pred_table, prediction_error, 4) +
+               energy_mean)) /
+        sqrtf(fixed_mean_energy);
+
+    // update quantified prediction error energy history
+    memmove(&prediction_error[0], &prediction_error[1],
+            3 * sizeof(prediction_error[0]));
+    prediction_error[3] = 20.0 * log10f(fixed_gain_factor);
+
+    return val;
+}
diff --git a/libavcodec/acelp_pitch_delay.h b/libavcodec/acelp_pitch_delay.h
index 2504a9e3dd..42307d2bba 100644
--- a/libavcodec/acelp_pitch_delay.h
+++ b/libavcodec/acelp_pitch_delay.h
@@ -220,4 +220,18 @@ int16_t ff_acelp_decode_gain_code(
     int subframe_size,
     int max_pred_order);
 
+/**
+ * Calculate fixed gain (part of section 6.1.3 of AMR spec)
+ *
+ * @param fixed_gain_factor gain correction factor
+ * @param fixed_energy decoded algebraic codebook vector energy
+ * @param prediction_error vector of the quantified predictor errors of
+ *        the four previous subframes. It is updated by this function.
+ * @param energy_mean desired mean innovation energy
+ * @param pred_table table of four moving average coefficients
+ */
+float ff_amr_set_fixed_gain(float fixed_gain_factor, float fixed_mean_energy,
+                            float *prediction_error, float energy_mean,
+                            const float *pred_table);
+
 #endif /* AVCODEC_ACELP_PITCH_DELAY_H */