Painting How To: Sizing Mats

For the following discussion about sizing mats, I’m going to talk about mats for a half-sheet (15″x22″) of 140# watercolor paper. That’s the paper I and most other artists use.  For those of you who paint with other types and sizes of paper, you’ll end up with different-size mats.  For those of you who may not even mat your paintings, you’ll most likely not need to know what I’m going to discuss, but you may find it informative, and perhaps useful.  It could save you time and money.  It may also get you thinking about different ways to go about matting… and framing.  You might identify better ways to present your work.

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Mats are designed to overlap a painting.  Most watercolor artists use a single mat with their paintings.  (But even if you double-mat your painting, the same principles apply.) Your mat needs to overlap all sides of your painting by at least 1/4″. If you cut your watercolor paper and mats, but can’t do it accurately, I’d recommend increasing the overlap to 1/2″.  Since I paint out to the edge of my paper, I don’t want this overlap to be any more than necessary, since I want people to be able to see as much of my painting as possible! So, the mat opening for my paintings needs to be as large as possible. For the standard half-sheet (15″x22″) 140# watercolor paper, overlapping it 1/4″ on all four sides produces a 14 1/2″ x 21 1/2″ mat opening

The size of the mat opening not only determines the overall dimensions of the mat, but also the frame. The reverse is also true. If you’ve already selected a frame, its nominal dimensions not only determine the overall dimensions of your mat, but also the actual dimensions of the mat opening… and therefore the size of your painting… at least that portion of it that you can see!  This being the case, you certainly want to make sure your frame is at least 15″ x 22″, large enough to accept the entire painting.  But if your painting was exactly this size, you’d end up with no mat.

Most artists want to have mats that complement their paintings. To achieve this objective, you need to know the width of all four borders for the mat so you can determine how much larger the frame has to be to accommodate the painting itself… assuming the frame is rectangular. Sizes of frames depend on the size of the borders that artist select for their mats. (If you don’t use a mat with your painting, you can skip the following discussion.)

If you choose identical 1″ wide borders for all four sides of the mat for a half-sheet painting, the overall dimensions will be:

1″ + 1″ + 14 1/2″ = 16 1/2″ and 1″ + 1 + 21 1/2″ = 23 1/2″

If you choose 2″ wide borders, the dimensions of the mat will be 18 1/2″ x 25 1/2″. Overall dimensions of mats with 3″ wide borders would be 20 1/2″ x 27 1/2″, and so on. I guess you could make the borders as wide as want! (But, of course, the wider the borders of the mat, the larger and more expensive is the frame). If this were the case, consider making borders an integral part of the painting!  Some artists do!

Decisions about widths of borders for mats determine not only the overall mat dimensions but also the nominal dimensions of frames. If you’re willing to pay the additional cost of custom frames, that’s all you need to know.  Simply decide on widths of borders you want for your mat, and go to a custom frame shop.  But if you’re on a budget, check out the dimensions of standard frames to help you select dimensions of your mats. Knowing sizes of standard frames becomes even more important if you want a weighted bottom border for the mat… one that’s wider than those other three which are generally the same. I prefer a weighted/wider bottom border for half-sheet (15″x22″) and larger paintings, whereas smaller quarter-sheets don’t seem to need them.

Let’s examine the width of borders for a matted horizontal painting in a nominal 28″x22″ frame. To start off, you’ll need a 28″ wide mat.  When you subtract the width of the mat opening (calculated to be 21 1/2″), the resulting width of either side border is 3 1/4″, the same as for the top border.  The corresponding nominal height for a mat that fits this standard frame is 22″. Given a top border of 3 1/4″ and 14 1/2″ vertical opening for the mat, the width of the bottom border is 22″ (overall width) – 3 1/4″ (top border) – 14 1/2″ (mat opening) = 4 1/4″. This works, giving you a weighted bottom border that’s 1″ wider than the 3 1/4″ top and side borders… and overall dimensions of 22″x28″ mat which suit a nominal 22″x28″ frame (see Exhibit 37A).

Exhibit 37A – Horizontal Mat w/ Weighted Bottom Border for Halfsheet Painting

But if you take the same frame, rotate it 90 degrees for a vertical painting, you run into a problem. The mat made for the horizontal painting wont work. One side is 4 1/4″, while the opposite side, top and bottom borders are 3 1/4″. (See Exhibit 37B)

Exhibit 37B – Absurd Vertical Mat

So let’s change the borders of the mat for the vertical painting. The width and height of mat opening for either vertical or horizontal painting are the same: 14 1/2″ x 21 1/2″. What was once the width is now the height, and the side borders become: 22″ (overall width) – 14 1/2″ (mat opg)= 7 1/2″/2 (borders) = 3 3/4″. Since top and side borders will be the same width (3 3/4″), the bottom border becomes: 28″ (overall height) – 21 1/2″ (mat opg) – 3 3/4″ (top border)= 2 3/4″ (Exhibit 37C). 

Exhibit 37C – Another Absurd Vertical Mat

This doesn’t work! The bottom border is actually narrower than the side borders!  You can, of course, reverse top and bottom borders, placing the 3 3/4″ wide border at the bottom and the 2 3/4″ wide bottom border on top. Or you could make them both the same: 3 1/4″. But that would also be strange, since both side borders would be wider(3 3/4″) (Exhibit 37D).

Exhibit 37D - The Last Absurd Vertical Mat

Exhibit 37D – The Last Absurd Vertical Mat

Now you see why I gave up 22Wx28H mats/frames for vertical paintings.  What you need is a narrower mat. To do this, of course, you need a narrower frame.

I discovered such a frame, one not only narrower, but also of natural wood, similar in appearance to those I use for my half-sheet horizontal paintings, one made by IKEA™. I use it for all my vertical half-sheet paintings.  The actual depth of the frame is more… 1 1/2″ instead of 3/4″, but about the same width.  It goes by the name of “Ribba”. Nominal dimensions are 19 1/2″ x 27 1/2″, same as the overall mat dimensions.

Calculating side, top and bottom borders of mats in the same manner, the top and sides become 2 1/2″, and the weighted bottom, 3 1/2″ (Exhibit 37E).                   

Exhibit 37E – Perfect Vertical Mat

By doing this, you end up with different mats for 15×22 vertical and horizontal paintings, and must keep two different size mats and frames on hand: 22×28 for horizontal, 19 1/2″ x 27 1/2″ for vertical paintings.

Standard 16″x20″ mats/frames work for 11″x15″ quarter-sheet paintings, whether vertical or horizontal, since you don’t need a weighted bottom mat for these smaller paintings. All borders can be 2 3/4″. If all sides of the mat being equal, you can use the same 16×20 mat and frame for any vertical or horizontal painting. That saves you money!

For full-sheet paintings (22″x30″) I’ve selected a different type of framing system: namely, metal frames that can be disassembled. I did this because it was much easier and less costly to ship these disassembled frame components in my suitcase. This gives me flexibility in sizing bottom-weighted mats for full-sheets of 140# or 300# watercolor paper oriented vertically or horizontally. I simply had to decide what the border widths should be.  For horizontal paintings, I chose 3 1/4″ for top and side borders, 4 1/4″ for the bottom.  The overall dimensions for both mat and frame thus become 29″Hx36″W.  For full-sheet vertical paintings I chose 2 3/4″ for the top and side borders, and 4 3/4″ for the bottom.  I kept the height of the frame and mat at 36″, same as the width for horizontal paintings, but the width for such vertical paintings becomes 27″ (rather than 29”) because I chose narrower side borders.  The longest dimension of mats/frames for any of my full-sheet paintings is therefore 36″, easy to remember when filling out exhibit entry forms, but also critical when it comes to ordering proper matting and framing materials.

Douglas Stenhouse also wrote a book about watercolor painting.

“I decided to write about how I paint, not only to share my observations with others, but also, frankly, to do some self- examination. I wish I had done this earlier in my life! But then, how was I to know I’d benefit from doing so, certainly at a time when I had no aspirations of becoming a professional artist.” 

To learn more about Doug’s Watercolor Painting Book, click here. To purchase a signed copy, use the paypal link below.


Douglas Simms Stenhouse, watercolor artist, transparent water color art, watercolor painter, painting with water colors