Calculate a cell size for flowlayout in uicollectionview , assuming you want square cells and have 1pt spacing all around.

collectionviewcontroller

- (void)viewWillLayoutSubviews;
{
    [super viewWillLayoutSubviews];
    
    UICollectionViewFlowLayout *flowLayout = (id)self.collectionViewLayout;
    // really should get this value from the Photo class, but for now max thumb of 110px.
    CGFloat cellSize = [self calculateCellSizeForMaxImageWidth:110];
    flowLayout.itemSize = CGSizeMake(cellSize, cellSize);
  
}

- (CGFloat)calculateCellSizeForMaxImageWidth: (NSInteger)maxWidth{
    
    UIScreen *mainScreen = [UIScreen mainScreen];
    NSInteger   numCells = ceil(mainScreen.bounds.size.width/maxWidth);
    NSLog(@"Recommended number of columns %li", (long)numCells);
    // this assumes 1pt spacing, insets and cells. 
    CGFloat cellSize =  (mainScreen.bounds.size.width -(numCells+1))/numCells;
    NSLog(@"Recommended cell size %f", cellSize);
    return cellSize;
    
}