diff --git a/bitcoin/feerate.c b/bitcoin/feerate.c
index 696ec4b39..c6b37c17c 100644
--- a/bitcoin/feerate.c
+++ b/bitcoin/feerate.c
@@ -1,4 +1,5 @@
 #include <bitcoin/feerate.h>
+#include <bitcoin/varint.h>
 #include <limits.h>
 #include <stdlib.h>
 
@@ -38,3 +39,12 @@ const char *feerate_style_name(enum feerate_style style)
 	}
 	abort();
 }
+
+size_t common_weight(size_t num_inputs, size_t num_outputs)
+{
+	/*(nVersion + num inputs + num outputs + locktime) * 4
+	 *         + SegWit marker + SegWit flag */
+	return (4 + varint_size(num_inputs) +
+		varint_size(num_outputs) + 4) * 4
+		+ 1 + 1;
+}
diff --git a/bitcoin/feerate.h b/bitcoin/feerate.h
index 43bec2118..6190625aa 100644
--- a/bitcoin/feerate.h
+++ b/bitcoin/feerate.h
@@ -34,6 +34,11 @@
  */
 #define FEERATE_FLOOR 253
 
+/*
+ * This is the net common weight of a transaction.
+ */
+size_t common_weight(size_t num_inputs, size_t num_outputs);
+
 enum feerate_style {
 	FEERATE_PER_KSIPA,
 	FEERATE_PER_KBYTE