diff --git a/drivers/soft_spi/include/soft_spi_params.h b/drivers/soft_spi/include/soft_spi_params.h
index 78a2425766..f17f8be93f 100644
--- a/drivers/soft_spi/include/soft_spi_params.h
+++ b/drivers/soft_spi/include/soft_spi_params.h
@@ -48,6 +48,11 @@ static soft_spi_conf_t soft_spi_config[] = {
     SOFT_SPI_PARAMS,
 };
 
+/**
+ * @brief   Number of software SPI buses
+ */
+#define SOFT_SPI_NUMOF             ARRAY_SIZE(soft_spi_config)
+
 #ifdef __cplusplus
 }
 #endif
diff --git a/drivers/soft_spi/soft_spi.c b/drivers/soft_spi/soft_spi.c
index 8786b4bcdb..6a7cff9698 100644
--- a/drivers/soft_spi/soft_spi.c
+++ b/drivers/soft_spi/soft_spi.c
@@ -34,7 +34,7 @@
 /**
  * @brief   Allocate one lock per SPI device
  */
-static mutex_t locks[sizeof soft_spi_config];
+static mutex_t locks[SOFT_SPI_NUMOF];
 
 static inline bool soft_spi_bus_is_valid(soft_spi_t bus)
 {