Slide sizes: OPF presets and PowerPoint presets

OPF sets one slide size per deck, in design.dimensions (or the resolved theme's dimensions). The value is a preset name ("16:9", or { "preset": "16:9" }) or a custom size in inches ({ "widthInches": 10, "heightInches": 7.5 }). A custom width or height overrides the preset's. A slide's own design cannot set a size, because a PPTX has one slide size (schema reference).

This page lists the exact size of every preset, which PowerPoint size each one exports as, and where an OPF name and a PowerPoint preset with a similar name differ. The values come from:

• resolveCanvasDimensions in packages/javascript/src/composition.ts, which every engine uses;
• the p:sldSz writer and reader in @openpresentation/opf-pptx (src/index.js);
• PowerPoint 16.0 build 20430, which reported its own preset sizes in the native run FF-27 (opf#323, opf#354).

Units#

• 1 inch = 914,400 EMU = 72 pt = 96 OPF reference pixels. 1 pt = 12,700 EMU.
• OPF composes every slide at 96 reference pixels per inch. A 13.333 x 7.5 in slide is a 1280 x 720 canvas, and a 10 x 7.5 in slide is 960 x 720. Text sizes are physical (points), so the same content wraps differently on a smaller slide.
• The PPTX p:sldSz element stores cx and cy in EMU. The exporter writes Math.round(inches x 914400).

OPF presets#

• widescreen is an alias of 16:9, and standard an alias of 4:3. Each pair exports the same bytes.
• No design.dimensions and no theme size means 16:9.
• letter and a4 are the paper sizes. a4 is rounded to 11.69 x 8.27 in. ISO A4 is 297 x 210 mm (11.6929 x 8.2677 in, 10692000 x 7560000 EMU), so the a4 slide is about 0.07 mm narrower and 0.06 mm taller than the paper.
• 1:1, 4:5 and 9:16 keep the widescreen short edge of 7.5 in.

PowerPoint presets#

PowerPoint's Design > Slide Size > Custom Slide Size dialog offers named sizes under "Slides sized for". The table lists the ones the native run read back, and the OPF preset that matches each one.

What differs:

• PowerPoint's paper presets are not the paper. "Letter Paper" and "A4 Paper" are slide areas that print on that paper with margins: 10 x 7.5 in on letter paper (11 x 8.5 in) and 10.833 x 7.5 in on A4 paper (11.69 x 8.27 in). OPF's letter and a4 are the full paper sizes, which suit print and PDF output. A deck that must match PowerPoint's "Letter Paper" or "A4 Paper" slide exactly uses custom widthInches and heightInches.
• "Widescreen" and "On-screen Show (16:9)" have the same shape at different sizes. Widescreen (13.333 x 7.5 in) has been PowerPoint's size for new decks since PowerPoint 2013, and it is what OPF's 16:9 writes. "On-screen Show (16:9)" is the older 10 x 5.625 in size. A deck at that size scales to Widescreen without changing shape, but text in points is relatively larger on the smaller slide, so it does not lay out the same.
• "On-screen Show (4:3)" and "(16:10)" are the same sizes as OPF's 4:3 and 16:10.
• PowerPoint's other presets (Ledger, B5, 35 mm Slides, Overhead, Banner) have no OPF preset. Use custom inches.

OPF kept its values in 2026-10 (opf#365, vetoable): the schema says a preset chooses both aspect ratio and physical size, and changing letter, a4 or 16:9 now would move every existing deck and golden for no fidelity gain.

Export to PPTX#

• @openpresentation/opf-pptx writes p:sldSz with the EMU values in the first table, and p:notesSz with the width and height swapped. It writes no type attribute, so the size is custom, the default of sldSz@type in ECMA-376. PowerPoint reports its own Widescreen size the same way: a new deck reads PageSetup.SlideSize = 7 (ppSlideSizeCustom), 960 x 540 pt.
• PowerPoint therefore shows an exported 4:3 or 16:10 deck as a custom size with the right dimensions, not as the "On-screen Show" preset. The size is the same, only the preset label differs.
• A deck whose slides resolve to different sizes (a slide-level theme with other dimensions) fails export with mixed-slide-dimensions, and validate warns first with opf/slide-theme-dimensions.
• ECMA-376 limits cx and cy to 914,400 to 51,206,400 EMU (1 to 56 in). The OPF schema only requires a positive size, so validate warns with opf/slide-size-out-of-range (a format warning, so valid does not change) when the deck's custom size has a side outside that range. It reads design.dimensions, or the resolved theme's dimensions when the deck sets none, and converts inches to EMU the way the exporter does (Math.round(inches x 914400)), so exactly 1 in and 56 in pass. The presets are all in range. The exporter does not check the range: it still writes the p:sldSz it is given, which does not conform to the standard when the size is outside 1 to 56 in. Run validate first; PowerPoint's own behaviour at the limits was not measured.

Import from PPTX#

• fromPptx reads p:sldSz cx and cy and sets design.dimensions to { widthInches: cx / 914400, heightInches: cy / 914400 }, unrounded. It does not match the size against the preset table and ignores the type attribute. A PowerPoint "Letter Paper" deck imports as { "widthInches": 10, "heightInches": 7.5 }, which composes on the same canvas as 4:3; an "On-screen Show (16:9)" deck imports as { "widthInches": 10, "heightInches": 5.625 } (a 960 x 540 canvas).
• A package that toPptx wrote carries the source document's provenance. While p:sldSz is unchanged, import restores the authored value ("a4", "widescreen", { "preset": "9:16" }) instead of the inches. If the size was changed in PowerPoint, the observed inches are imported and design.dimensions is reported as changed. A deck that stated no size and still has the exported one imports with no design.dimensions.
• A package with no p:sldSz imports with no design.dimensions, and its slides are read at the widescreen size.

Sources#

• ECMA-376 / ISO/IEC 29500-1, PresentationML: §19.2.1.39 sldSz (Presentation Slide Size; cx, cy, and type with default custom), §19.7.17 ST_SlideSizeCoordinate (914400 to 51206400 EMU) and §19.7.18 ST_SlideSizeType (screen4x3, screen16x9, screen16x10, letter, A4, A3, ledger, B4ISO, B5ISO, B4JIS, B5JIS, 35mm, overhead, banner, hagakiCard, custom).
• Microsoft Open XML SDK: SlideSize and SlideSizeValues.
• PowerPoint VBA: PpSlideSizeType.
• Microsoft Support: Change the size of your slides (Widescreen is the default for new presentations, and the list of "Slides sized for" presets).
• Native measurements: FF-27 slide-size presets (sizes-compare.md).