diff --git a/examples/capture/main.rs b/examples/capture/main.rs
index 96bd1d7ff8..d0757c0c64 100644
--- a/examples/capture/main.rs
+++ b/examples/capture/main.rs
@@ -87,7 +87,7 @@ fn main() {
     // Write the buffer as a PNG
     output_buffer.map_read_async(
         0,
-        (size * size) as u64 * size_of::<u32>() as u64,
+        (size * size) as usize * size_of::<u32>(),
         move |result: wgpu::BufferMapAsyncResult<&[u8]>| {
             let mut png_encoder = png::Encoder::new(File::create("red.png").unwrap(), size, size);
             png_encoder.set_depth(png::BitDepth::Eight);
diff --git a/examples/hello-compute/main.rs b/examples/hello-compute/main.rs
index 7657a910a5..d3bdf388a1 100644
--- a/examples/hello-compute/main.rs
+++ b/examples/hello-compute/main.rs
@@ -92,7 +92,7 @@ fn main() {
 
     queue.submit(&[encoder.finish()]);
 
-    staging_buffer.map_read_async(0, size, |result: wgpu::BufferMapAsyncResult<&[u32]>| {
+    staging_buffer.map_read_async(0, numbers.len(), |result: wgpu::BufferMapAsyncResult<&[u32]>| {
         if let Ok(mapping) = result {
             println!("Times: {:?}", mapping.data);
         }
diff --git a/src/lib.rs b/src/lib.rs
index be4e10eea0..70526d034f 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -906,7 +906,7 @@ where
 }
 
 impl Buffer {
-    pub fn map_read_async<T, F>(&self, start: BufferAddress, size: BufferAddress, callback: F)
+    pub fn map_read_async<T, F>(&self, start: BufferAddress, count: usize, callback: F)
     where
         T: 'static + FromBytes,
         F: FnOnce(BufferMapAsyncResult<&[T]>) + 'static,
@@ -936,6 +936,8 @@ impl Buffer {
             }
         }
 
+        let size = (count * std::mem::size_of::<T>()) as BufferAddress;
+
         let user_data = Box::new(BufferMapReadAsyncUserData {
             size,
             callback,
@@ -951,7 +953,7 @@ impl Buffer {
         );
     }
 
-    pub fn map_write_async<T, F>(&self, start: BufferAddress, size: BufferAddress, callback: F)
+    pub fn map_write_async<T, F>(&self, start: BufferAddress, count: usize, callback: F)
     where
         T: 'static + AsBytes + FromBytes,
         F: FnOnce(BufferMapAsyncResult<&mut [T]>) + 'static,
@@ -981,6 +983,8 @@ impl Buffer {
             }
         }
 
+        let size = (count * std::mem::size_of::<T>()) as BufferAddress;
+
         let user_data = Box::new(BufferMapWriteAsyncUserData {
             size,
             callback,