diff --git a/Makefile b/Makefile
index 88ba7a3c514c4f..15990ff3122eb6 100644
--- a/Makefile
+++ b/Makefile
@@ -1340,6 +1340,7 @@ THIRD_PARTY_SOURCES += sha1collisiondetection/%
 THIRD_PARTY_SOURCES += sha1dc/%
 
 UNIT_TEST_PROGRAMS += t-basic
+UNIT_TEST_PROGRAMS += t-mem-pool
 UNIT_TEST_PROGRAMS += t-strbuf
 UNIT_TEST_PROGS = $(patsubst %,$(UNIT_TEST_BIN)/%$X,$(UNIT_TEST_PROGRAMS))
 UNIT_TEST_OBJS = $(patsubst %,$(UNIT_TEST_DIR)/%.o,$(UNIT_TEST_PROGRAMS))
diff --git a/mem-pool.c b/mem-pool.c
index c34846d176c886..c7d62560201984 100644
--- a/mem-pool.c
+++ b/mem-pool.c
@@ -89,9 +89,7 @@ void *mem_pool_alloc(struct mem_pool *pool, size_t len)
 	struct mp_block *p = NULL;
 	void *r;
 
-	/* round up to a 'GIT_MAX_ALIGNMENT' alignment */
-	if (len & (GIT_MAX_ALIGNMENT - 1))
-		len += GIT_MAX_ALIGNMENT - (len & (GIT_MAX_ALIGNMENT - 1));
+	len = DIV_ROUND_UP(len, GIT_MAX_ALIGNMENT) * GIT_MAX_ALIGNMENT;
 
 	if (pool->mp_block &&
 	    pool->mp_block->end - pool->mp_block->next_free >= len)
@@ -99,9 +97,9 @@ void *mem_pool_alloc(struct mem_pool *pool, size_t len)
 
 	if (!p) {
 		if (len >= (pool->block_alloc / 2))
-			return mem_pool_alloc_block(pool, len, pool->mp_block);
-
-		p = mem_pool_alloc_block(pool, pool->block_alloc, NULL);
+			p = mem_pool_alloc_block(pool, len, pool->mp_block);
+		else
+			p = mem_pool_alloc_block(pool, pool->block_alloc, NULL);
 	}
 
 	r = p->next_free;
diff --git a/t/unit-tests/t-mem-pool.c b/t/unit-tests/t-mem-pool.c
new file mode 100644
index 00000000000000..a0d57df7616fb6
--- /dev/null
+++ b/t/unit-tests/t-mem-pool.c
@@ -0,0 +1,31 @@
+#include "test-lib.h"
+#include "mem-pool.h"
+
+static void setup_static(void (*f)(struct mem_pool *), size_t block_alloc)
+{
+	struct mem_pool pool = { .block_alloc = block_alloc };
+	f(&pool);
+	mem_pool_discard(&pool, 0);
+}
+
+static void t_calloc_100(struct mem_pool *pool)
+{
+	size_t size = 100;
+	char *buffer = mem_pool_calloc(pool, 1, size);
+	for (size_t i = 0; i < size; i++)
+		check_int(buffer[i], ==, 0);
+	if (!check(pool->mp_block != NULL))
+		return;
+	check(pool->mp_block->next_free != NULL);
+	check(pool->mp_block->end != NULL);
+}
+
+int cmd_main(int argc, const char **argv)
+{
+	TEST(setup_static(t_calloc_100, 1024 * 1024),
+	     "mem_pool_calloc returns 100 zeroed bytes with big block");
+	TEST(setup_static(t_calloc_100, 1),
+	     "mem_pool_calloc returns 100 zeroed bytes with tiny block");
+
+	return test_done();
+}