Calculate effective magnification, virtual image size, and viewing angle for screen magnifiers, loupes, and accessibility zoom settings.
The Screen Magnification Calculator determines the effective magnification, resulting image size, and optimal viewing distances for screen magnifiers, optical loupes, and digital zoom settings. Whether you're configuring accessibility zoom for low-vision users or selecting a magnifying lens, this tool shows exactly how large content appears.
Magnification is the ratio of the apparent size of an object through the magnifier to its size with the naked eye. A 2× magnifier makes text appear twice as tall. But effective magnification depends on the relationship between focal length, viewing distance, and screen DPI — not just a simple multiplier.
This calculator handles both digital magnification (percentage zoom in operating systems) and optical magnification (diopter-based lenses). It computes the resulting text size in millimeters, the angular subtense at the viewer's eye, and whether the result meets common readability thresholds for various visual acuity levels. That makes it easier to compare accessibility zoom settings with optical aids on the same practical scale.
Configure the right magnification level for accessibility, choose optical magnifiers for low-vision assistance, or understand how screen zoom affects text readability. It is useful when you need to compare readability at different viewing distances, zoom levels, or lens strengths before changing a setup. The output keeps those tradeoffs visible in physical size as well as magnification factor.
Digital: Magnified Size = Original Size × (Zoom% / 100). Optical: Magnification = Diopters / 4 + 1 (for 25cm reference). Angular Subtense (arcminutes) = atan(Size / Distance) × (180/π) × 60.
Result: 7.0 mm magnified, 60.2 arcminutes at 40cm
3.5mm text at 200% zoom becomes 7.0mm. At 40cm viewing distance, this subtends 60.2 arcminutes — well above the 5 arcminute threshold for 20/20 vision.
Visual acuity is measured as a fraction: 20/20 means you can resolve detail at 20 feet that a standard eye resolves at 20 feet. A person with 20/80 vision sees at 20 feet what a standard eye sees at 80 feet — they need 4× magnification (80÷20) to achieve effective 20/20 resolution.
The key metric is angular subtense: how large an angle the text or object occupies at the viewer's eye. Standard 12pt text at 40cm subtends about 17 arcminutes. For 20/20 vision, the minimum resolvable detail is 1 arcminute (letters need about 5 arcminutes to be recognizable).
Digital magnification scales pixels. At 200% on a 1080p screen, you effectively see a 960×540 viewport — half the information fits on screen. However, on a 4K screen at 200%, you still see 1920×1080 worth of content, which is why high-DPI displays pair so well with accessibility zoom.
Optical magnification uses lenses to bend light, producing a larger virtual image without sacrificing resolution. The tradeoff is field of view: higher optical power means a smaller area is visible at once, requiring more head/eye movement.
WCAG 2.1 Success Criterion 1.4.4 requires that text can be resized up to 200% without loss of content or functionality. This means every website should work correctly at 200% browser zoom — a form of digital magnification that's essential for millions of users with low vision.
It depends on remaining visual acuity. A person with 20/80 vision needs roughly 4× magnification to read standard print (which normally requires 20/20). Divide the denominator of their acuity by 20 to estimate needed magnification.
A diopter is the reciprocal of the focal length in meters. A +4 diopter lens has a focal length of 25cm. For magnification, the formula M = D/4 + 1 assumes a standard 25cm reference distance.
Digital zoom scales the rendered image. At 200%, each pixel is drawn as a 2×2 block. This doubles the apparent size of everything but halves the effective screen resolution and information density.
1M print (1 meter font) is the standard used in low-vision assessment. At 1 meter distance, 1M text subtends 5 arcminutes — the threshold for 20/20 vision. Each multiple (2M, 4M, etc.) doubles the size.
Optical magnification preserves full resolution. Digital magnification is more convenient and adjustable but reduces effective resolution. For accessibility, digital zoom combined with high-DPI screens is often the best approach.
For normal vision at 40cm, text should subtend at least 15-20 arcminutes (about 1.7-2.3mm). For extended reading comfort, 25+ arcminutes is recommended. Low-vision users may need 60+ arcminutes.