Practical guides

How much electricity does an appliance use?

A 1,000 W heater running at full power for two hours a day uses 60 kWh in 30 days. At an illustrative $0.18 per kWh, that’s $10.80.

Interactive example

How to calculate reverse percentages and percentage change

Percentage changes depend on the starting value. Work through increases and decreases, reverse a change and see why equal percentages do not always cancel out.

What does a charge cost? Work from battery level to billed energy

A battery percentage is not a direct measurement of the energy you pay for. A useful estimate separates stored energy, input energy and the service’s billing basis.

Why a light package can have a higher billable weight

A package takes up space even when its contents are light. Understanding a shipping estimate requires comparing scale weight with volume under the selected service’s rules.

How to reduce photo file size

A photo can be large in pixel dimensions, file size or both. Start with the recipient’s limit, make a copy and compare the actual result. A smaller file is useful only if the details that matter are still clear.

Interactive example

Convert HEIC to JPG: what to check before sending

A form that rejects HEIC may accept a converted JPG of the same photo. Aim for a usable copy, checking the image itself as well as the file format.

Before sharing a photo: location data, EXIF and visible clues

A photo can disclose information through both metadata and pixels. Cleaning the file is useful, but reviewing what the image shows is a separate step.

4:3 vs 16:9: choosing an image aspect ratio

Separate aspect ratio from resolution. Learn when to resize, crop or add borders while keeping a photo’s proportions.

DPI vs PPI: choosing a print resolution

Connect pixel count, physical size and viewing distance. Check a ten-inch print with a worked 300 PPI example.

How to create a QR code for a link

Take a link through to a tested printed copy. Prepare a QR code for a menu, flyer or business card, place it in the layout and check it before ordering the print run.

Interactive example

Check text contrast without rounding away a failure

Colors that look different can still produce hard-to-read text. Measure the actual foreground-background pair, then consider text size, background conditions and interface states.

HTML special characters: when to escape text

Separate text from markup, avoid double encoding and understand why HTML entities cannot replace sanitization.

HTML whitespace: formatting code with care

Learn which spaces can change a rendered page and what to check after indenting an HTML fragment.

JSON key order: comparing objects and arrays

Separate property order from array order. Read difference paths and check types, exact numbers and duplicate keys.

How to prepare a PDF to send by email

Combine your documents and check file size and readability before sending.

Interactive example

How to extract text from a scanned PDF

A PDF can display words without containing text you can copy. Follow the route from identifying a scan to extracting useful text and checking recognition errors.

How to save only the PDF pages you need

Sharing a chapter or attachment should not require sending the entire document. The important part is selecting the right pages and checking exactly what your new file contains.

PDF to Word: choose editing or a visual page copy

Do you need to change the words or reproduce a page? Those are different goals. Your conversion mode determines what can actually be edited in the Word file.

How to make a video smaller without losing useful detail

A smaller video is useful only if people can still understand it. Start with the recipient’s limit and compare a real output rather than relying on a promised percentage reduction.

Turn audio into a transcript you have actually checked

A fluent transcript can still contain a mistake that reverses the meaning. Finish the job by comparing the text with the recording, rather than accepting the first automatic result.

How to write scientific notation, step by step

Move the decimal point, choose the exponent sign and reconstruct the original number. Examples cover large, small and negative values.

Absolute vs relative error: calculating percent error

Choose a reference and keep measurement units consistent. Distinguish a percentage deviation from measurement uncertainty.

Mbps vs MB/s: understanding download time

Distinguish bits, bytes, decimal and binary units and advertised speed. Compare an estimated transfer time with a real download.

How to read four-band and five-band resistors

Find the reading direction, separate digits, multiplier and tolerance and check a worked 4.7 kΩ resistor example.