diff --git a/include/fbgfx/surface.h b/include/fbgfx/surface.h
index f9c77c9..c800325 100644
--- a/include/fbgfx/surface.h
+++ b/include/fbgfx/surface.h
@@ -27,6 +27,7 @@ typedef struct SURFACE {
 	int bytes_per_pixel;
 	int width;
 	int height;
+	uint32_t top_left_index;
 	int x_inc;
 	int y_inc;
 	RECT clip_region;
@@ -83,10 +84,7 @@ static inline uint32_t surface_get_buffer_size(const SURFACE *surface) {
 }
 
 static inline uint32_t surface_get_index_of(const SURFACE *surface, int x, int y) {
-	if (surface->flags & SURFACE_FLAGS_SIDEWAYS_BUFFER)
-		return (uint32_t)((x * surface->height) + (surface->height - y - 1)) * surface->bytes_per_pixel;
-	else
-		return (uint32_t)(x + (y * surface->width)) * surface->bytes_per_pixel;
+	return (uint32_t)(surface->top_left_index + (surface->x_inc * x) + (surface->y_inc * y));
 }
 
 static inline COLOR_COMPONENT* surface_get_pointer_to(const SURFACE *surface, int x, int y) {
diff --git a/src/fbgfx/surface.c b/src/fbgfx/surface.c
index b829683..923f91f 100644
--- a/src/fbgfx/surface.c
+++ b/src/fbgfx/surface.c
@@ -44,6 +44,13 @@ void surface_swap_framebuffers() {
 	*/
 }
 
+static uint32_t get_index(const SURFACE *surface, int x, int y) {
+	if (surface->flags & SURFACE_FLAGS_SIDEWAYS_BUFFER)
+		return (uint32_t)((x * surface->height) + (surface->height - y - 1)) * surface->bytes_per_pixel;
+	else
+		return (uint32_t)(x + (y * surface->width)) * surface->bytes_per_pixel;
+}
+
 void setup_initial_surface_properties(SURFACE *surface, int width, int height, SURFACE_FORMAT format, SURFACE_FLAGS flags) {
 	surface->width = width;
 	surface->height = height;
@@ -54,10 +61,11 @@ void setup_initial_surface_properties(SURFACE *surface, int width, int height, S
 
 	surface->clip_region = rect_create(0, 0, surface->width, surface->height);
 
-	// lazy calculation for the win
-	uint32_t reference_index = surface_get_index_of(surface, 0, 0);
-	surface->x_inc = surface_get_index_of(surface, 1, 0) - reference_index;
-	surface->y_inc = surface_get_index_of(surface, 0, 1) - reference_index;
+	// lazy calculations for the win
+	uint32_t reference_index = get_index(surface, 0, 0);
+	surface->top_left_index = reference_index;
+	surface->x_inc = get_index(surface, 1, 0) - reference_index;
+	surface->y_inc = get_index(surface, 0, 1) - reference_index;
 }
 
 SURFACE* surface_create(int width, int height, SURFACE_FORMAT format, SURFACE_FLAGS flags) {
diff --git a/test/test_surfaces.c b/test/test_surfaces.c
index 0b7c218..cf4eb0e 100644
--- a/test/test_surfaces.c
+++ b/test/test_surfaces.c
@@ -124,7 +124,38 @@ void test_coords_and_offsets(SURFACE_FLAGS flags) {
 	surface_destroy(surface);
 }
 
+void test_increments(SURFACE_FLAGS flags) {
+	SURFACE *surface = surface_create(1024, 1024, SURFACE_FORMAT_RGBA, flags);
+	assert(surface != NULL);
+
+	int x = 0;
+	int y = 0;
+	uint32_t index = surface_get_index_of(surface, x, y);
+
+	// this is a kind of silly test, but meh, a nice simple-ish validation
+	// over a wide range of possible coordinates
+
+	for (; y < surface->height; ++y) {
+		uint32_t temp = index;
+
+		for (; x < surface->width; ++x) {
+			uint32_t this_index = surface_get_index_of(surface, x, y);
+			assert(index == this_index);
+			index += surface->x_inc;
+		}
+
+		// because the previous x_inc increments make this unnecessary, but we want to test
+		// this increment too ...
+		index = temp;
+		index += surface->y_inc;
+	}
+
+	surface_destroy(surface);
+}
+
 int main(int argc, char **argv) {
+	test_increments(SURFACE_FLAGS_NONE);
+	test_increments(SURFACE_FLAGS_SIDEWAYS_BUFFER);
 	test_coords_and_offsets(SURFACE_FLAGS_NONE);
 	test_coords_and_offsets(SURFACE_FLAGS_SIDEWAYS_BUFFER);
 	test_rgba(SURFACE_FLAGS_NONE);