diff --git a/src/objects/mesh.nim b/src/objects/mesh.nim
index 046f3bb..4a7ccf0 100644
--- a/src/objects/mesh.nim
+++ b/src/objects/mesh.nim
@@ -420,21 +420,22 @@ proc ensure_capacity*(self: Mesh, extra_elements: int) =
     if self.data == nil:
         let stride = self.layout.stride
         assert stride != 0
-        self.load_from_va_ia @[newArrRef[float32](4*stride*extra_elements)]
+        let stride4 = stride div 4 # note: assuming layout.stride is aligned
+        self.load_from_va_ia @[newArrRef[float32](extra_elements * stride4)]
         self.data.num_indices[0] = 0
         return
     let varray = self.data.varrays[0]
-    let bytelen = varray.byteLen
+    let len = varray.len
     let current_index = self.data.num_indices[0]
     let stride = self.data.stride
     let index = current_index + extra_elements
-    let fpos = (stride div 4) * index
-    if fpos >= bytelen:
-        var cap = bytelen * 2
+    let fpos = stride * index * 4
+    if fpos >= len:
+        var cap = len * 2
         while fpos >= cap:
             cap *= 2
         var va = newArrRef[float32](cap)
-        copyMem(va.toPointer, varray.toPointer, bytelen)
+        copyMem(va.toPointer, varray.toPointer, len * sizeof(float32))
         self.load_from_va_ia @[va]
         self.data.num_indices[0] = current_index