diff --git a/py/modstruct.c b/py/modstruct.c
index b53dd51836..46d0ad004d 100644
--- a/py/modstruct.c
+++ b/py/modstruct.c
@@ -34,6 +34,7 @@
 #include "objtuple.h"
 #include "objstr.h"
 #include "binary.h"
+#include "parsenum.h"
 
 #if MICROPY_ENABLE_MOD_STRUCT
 
@@ -56,9 +57,26 @@ STATIC char get_fmt_type(const char **fmt) {
     return t;
 }
 
+STATIC machine_uint_t get_fmt_num(const char **p) {
+    const char *num = *p;
+    uint len = 1;
+    while (unichar_isdigit(*++num)) {
+        len++;
+    }
+    machine_uint_t val = (machine_uint_t)MP_OBJ_SMALL_INT_VALUE(mp_parse_num_integer(*p, len, 10));
+    *p = num;
+    return val;
+}
+
 STATIC uint calcsize_items(const char *fmt) {
-    // TODO
-    return strlen(fmt);
+    uint cnt = 0;
+    while (*fmt) {
+        // TODO supports size spec only for "s"
+        if (!unichar_isdigit(*fmt++)) {
+            cnt++;
+        }
+    }
+    return cnt;
 }
 
 STATIC mp_obj_t struct_calcsize(mp_obj_t fmt_in) {
@@ -67,9 +85,23 @@ STATIC mp_obj_t struct_calcsize(mp_obj_t fmt_in) {
     machine_uint_t size;
     for (size = 0; *fmt; fmt++) {
         uint align;
-        int sz = mp_binary_get_size(fmt_type, *fmt, &align);
+        machine_uint_t cnt = 1;
+        if (unichar_isdigit(*fmt)) {
+            cnt = get_fmt_num(&fmt);
+        }
+        if (cnt > 1) {
+            // TODO: count spec support only for string len
+            assert(*fmt == 's');
+        }
+
+        machine_uint_t sz;
+        if (*fmt == 's') {
+            sz = cnt;
+        } else {
+            sz = (machine_uint_t)mp_binary_get_size(fmt_type, *fmt, &align);
+        }
         // TODO
-        assert(sz != -1);
+        assert(sz != (machine_uint_t)-1);
         // Apply alignment
         size = (size + align - 1) & ~(align - 1);
         size += sz;
@@ -89,7 +121,22 @@ STATIC mp_obj_t struct_unpack(mp_obj_t fmt_in, mp_obj_t data_in) {
     byte *p = bufinfo.buf;
 
     for (uint i = 0; i < size; i++) {
-        mp_obj_t item = mp_binary_get_val(fmt_type, *fmt++, &p);
+        machine_uint_t sz = 1;
+        if (unichar_isdigit(*fmt)) {
+            sz = get_fmt_num(&fmt);
+        }
+        if (sz > 1) {
+            // TODO: size spec support only for string len
+            assert(*fmt == 's');
+        }
+        mp_obj_t item;
+        if (*fmt == 's') {
+            item = mp_obj_new_bytes(p, sz);
+            p += sz;
+            fmt++;
+        } else {
+            item = mp_binary_get_val(fmt_type, *fmt++, &p);
+        }
         res->items[i] = item;
     }
     return res;
@@ -106,7 +153,29 @@ STATIC mp_obj_t struct_pack(uint n_args, mp_obj_t *args) {
     memset(p, 0, size);
 
     for (uint i = 1; i < n_args; i++) {
-        mp_binary_set_val(fmt_type, *fmt++, args[i], &p);
+        machine_uint_t sz = 1;
+        if (unichar_isdigit(*fmt)) {
+            sz = get_fmt_num(&fmt);
+        }
+        if (sz > 1) {
+            // TODO: size spec support only for string len
+            assert(*fmt == 's');
+        }
+
+        if (*fmt == 's') {
+            mp_buffer_info_t bufinfo;
+            mp_get_buffer_raise(args[i], &bufinfo, MP_BUFFER_READ);
+            machine_uint_t to_copy = sz;
+            if (bufinfo.len < to_copy) {
+                to_copy = bufinfo.len;
+            }
+            memcpy(p, bufinfo.buf, to_copy);
+            memset(p + to_copy, 0, sz - to_copy);
+            p += sz;
+            fmt++;
+        } else {
+            mp_binary_set_val(fmt_type, *fmt++, args[i], &p);
+        }
     }
     return res;
 }
diff --git a/tests/basics/struct1.py b/tests/basics/struct1.py
index 3a05c85f0b..b114a789b5 100644
--- a/tests/basics/struct1.py
+++ b/tests/basics/struct1.py
@@ -16,3 +16,8 @@ print(struct.pack(">b", 1))
 
 print(struct.pack("<bI", -128, 256))
 print(struct.pack(">bI", -128, 256))
+
+print(struct.calcsize("100sI"))
+print(struct.calcsize("97sI"))
+print(struct.unpack("<6sH", b"foo\0\0\0\x12\x34"))
+print(struct.pack("<6sH", b"foo", 10000))