diff --git a/c_src/sema_nif.cpp b/c_src/sema_nif.cpp
index ae1f651..1e1318a 100644
--- a/c_src/sema_nif.cpp
+++ b/c_src/sema_nif.cpp
@@ -3,6 +3,7 @@
 #include <atomic>
 #include <mutex>
 #include <map>
+#include <cassert>
 
 #ifdef TRACE
 #include <iostream>
@@ -99,6 +100,9 @@ struct sema {
         return map_ret;
     }
 
+    unsigned capacity() const     { return max; }
+    unsigned capacity(unsigned n) { auto old=max; max=n; return old; }
+
     ERL_NIF_TERM try_to_take(ErlNifEnv *env, const ErlNifPid& pid, unsigned n) {
         unsigned x = cnt.load(std::memory_order_relaxed);
         while (x + n <= max) {
@@ -272,8 +276,7 @@ create(ErlNifEnv *env, int argc, const ERL_NIF_TERM argv[]) {
 
 static ERL_NIF_TERM
 info(ErlNifEnv *env, int argc, const ERL_NIF_TERM argv[]) {
-    if (argc != 1)
-        return enif_make_badarg(env);
+    assert(argc == 1);
 
     sema *res = nullptr;
     if (!enif_get_resource(env, argv[0], SEMA, (void **)&res))
@@ -285,6 +288,21 @@ info(ErlNifEnv *env, int argc, const ERL_NIF_TERM argv[]) {
     return res->info(env);
 }
 
+static ERL_NIF_TERM
+capacity(ErlNifEnv *env, int argc, const ERL_NIF_TERM argv[]) {
+    assert(argc >= 1 && argc <= 2);
+
+    sema *res = nullptr;
+    if (!enif_get_resource(env, argv[0], SEMA, (void **)&res) || !res)
+        return enif_make_badarg(env);
+
+    unsigned max = 0;
+    if (argc == 2 && !enif_get_uint(env, argv[1], &max))
+        return enif_make_badarg(env);
+        
+    return enif_make_uint(env, argc == 2 ? res->capacity(max) : res->capacity());
+}
+
 static ERL_NIF_TERM
 acquire(ErlNifEnv *env, int argc, const ERL_NIF_TERM argv[]) {
     if (argc < 1 || argc > 2)
@@ -357,13 +375,15 @@ release(ErlNifEnv *env, int argc, const ERL_NIF_TERM argv[]) {
 }
 
 static ErlNifFunc nif_funcs[] = {
-    {"create", 1, create},
-    {"info", 1, info},
-    {"acquire", 1, acquire},
-    {"acquire", 2, acquire},
-    {"release", 1, release},
-    {"release", 2, release},
-    {"release", 3, release}
+    {"create",   1, create},
+    {"info",     1, info},
+    {"capacity", 1, capacity},
+    {"capacity", 2, capacity},
+    {"acquire",  1, acquire},
+    {"acquire",  2, acquire},
+    {"release",  1, release},
+    {"release",  2, release},
+    {"release",  3, release}
 };
 
 ERL_NIF_INIT(sema_nif, nif_funcs, &load, nullptr, nullptr, nullptr);
diff --git a/src/sema_nif.erl b/src/sema_nif.erl
index 805cecd..3d6f67e 100644
--- a/src/sema_nif.erl
+++ b/src/sema_nif.erl
@@ -3,6 +3,8 @@
 -export([
     create/1,
     info/1,
+    capacity/1,
+    capacity/2,
     acquire/1,
     acquire/2,
     release/1,
@@ -13,6 +15,8 @@
 -nifs([
     create/1,
     info/1,
+    capacity/1,
+    capacity/2,
     acquire/1,
     acquire/2,
     release/1,
@@ -41,10 +45,11 @@
 
 -export_type([sema_ref/0, acquire_ret/0, release_ret/0]).
 
+% @doc Create a new semaphore with the given capacity
 -spec create(Max :: pos_integer()) -> sema_ref().
 create(_) -> not_loaded(?LINE).
 
-% get internal properties of the semaphore resource
+% @doc Get internal properties of the semaphore resource
 -spec info(Semaphore :: sema_ref()) ->
     #{
         % number of units acquired
@@ -56,23 +61,32 @@ create(_) -> not_loaded(?LINE).
     }.
 info(_) -> not_loaded(?LINE).
 
-% acquire resource unit for calling process, monitor process
+% @doc Get semaphore maximum capacity
+-spec capacity(Semaphore :: sema_ref()) -> pos_integer().
+capacity(_) -> not_loaded(?LINE).
+
+% @doc Set semaphore maximum capacity.
+% Return old capacity.
+-spec capacity(Semaphore :: sema_ref(), Max :: integer()) -> pos_integer().
+capacity(_, _Max) -> not_loaded(?LINE).
+
+% @doc Acquire resource unit for calling process, monitor process
 -spec acquire(Semaphore :: sema_ref()) -> Ret :: acquire_ret().
 acquire(_) -> not_loaded(?LINE).
 
-% acquire resource Cnt units for calling process, monitor process
+% @doc Acquire resource Cnt units for calling process, monitor process
 -spec acquire(Semaphore :: sema_ref(), Cnt :: pos_integer()) -> Ret :: acquire_ret().
 acquire(_, _) -> not_loaded(?LINE).
 
-% release resource unit acquired by calling process
+% @doc Release resource unit acquired by calling process
 -spec release(Semaphore :: sema_ref()) -> Ret :: release_ret().
 release(_) -> not_loaded(?LINE).
 
-% release resource unit(s) acquired by calling/another process
+% @doc Release resource unit(s) acquired by calling/another process
 -spec release(Semaphore :: sema_ref(), Arg :: pos_integer() | pid()) -> Ret :: release_ret().
 release(_, _) -> not_loaded(?LINE).
 
-% release resource units acquired by another process
+% @doc Release resource units acquired by another process
 -spec release(Semaphore :: sema_ref(), Cnt :: pos_integer(), Pid :: pid()) -> Ret :: release_ret().
 release(_, _, _) -> not_loaded(?LINE).
 
diff --git a/test/sema_tests.erl b/test/sema_tests.erl
index a1e1fc6..4a75706 100644
--- a/test/sema_tests.erl
+++ b/test/sema_tests.erl
@@ -6,6 +6,7 @@ basic_api_test() ->
     ?assertError(badarg, sema_nif:create(-1)),
 
     S = sema_nif:create(3),
+    ?assertEqual(3, sema_nif:capacity(S)),
     ?assertEqual(#{cnt => 0, dead => 0, max => 3}, sema_nif:info(S)),
 
     ?assertEqual({ok, 1}, sema_nif:acquire(S)),
@@ -65,6 +66,8 @@ basic_api_test() ->
     ?assertEqual({ok, 0}, sema_nif:release(S, 2)),
     ?assertEqual(#{cnt => 0, dead => 0, max => 3}, sema_nif:info(S)),
 
+    ?assertEqual(3, sema_nif:capacity(S, 5)),
+    ?assertEqual(5, sema_nif:capacity(S)),
     ok.
 
 gc_test() ->