diff --git a/src/include/lwip/opt.h b/src/include/lwip/opt.h
index 66bf9af2..02df3359 100644
--- a/src/include/lwip/opt.h
+++ b/src/include/lwip/opt.h
@@ -313,7 +313,7 @@
  * The formula expects settings to be either '0' or '1'.
  */
 #ifndef MEMP_NUM_SYS_TIMEOUT
-#define MEMP_NUM_SYS_TIMEOUT            (LWIP_TCP + IP_REASSEMBLY + LWIP_ARP + (2*LWIP_DHCP) + LWIP_AUTOIP + LWIP_IGMP + LWIP_DNS + PPP_SUPPORT + (LWIP_IPV6 ? (1 + LWIP_IPV6_REASS + LWIP_IPV6_MLD) : 0))
+#define MEMP_NUM_SYS_TIMEOUT            (LWIP_TCP + IP_REASSEMBLY + LWIP_ARP + (2*LWIP_DHCP) + LWIP_AUTOIP + LWIP_IGMP + LWIP_DNS + (PPP_SUPPORT*6*MEMP_NUM_PPP_PCB) + (LWIP_IPV6 ? (1 + LWIP_IPV6_REASS + LWIP_IPV6_MLD) : 0))
 #endif
 
 /**