How .ics and .vcf Files Work: Calendar Events and Contacts Explained

Two plain-text formats sit behind everyday actions you never think about: .ics is the file behind every “Add to Calendar” button, and .vcf (vCard) is the file behind every saved contact. Both are just text — but both have quirks that trip people up: escaping, line-folding, and iCalendar’s notorious exclusive all-day end date. Here’s how each works, with a browser-based ICS generator and VCF↔CSV converters that keep your data on your device.

The .ics calendar file

Open one and you’ll see a nested, line-based structure:

BEGIN:VCALENDAR
VERSION:2.0
PRODID:-//LazyTools//ICS Generator//EN
BEGIN:VEVENT
UID:abc123@lazytools.io
DTSTAMP:20240101T000000Z
DTSTART:20240603T093000
DTEND:20240603T103000
SUMMARY:Team Sync
LOCATION:Room 4
RRULE:FREQ=WEEKLY;COUNT=5
END:VEVENT
END:VCALENDAR

An event is a VEVENT block with a start (DTSTART), usually an end (DTEND), a SUMMARY (the title), and optional LOCATION, DESCRIPTION and a repeat rule (RRULE). Save that as .ics and any calendar app offers to add it. The wrapping VCALENDAR container carries the VERSION (always 2.0 for iCalendar) and a PRODID that names the software that wrote the file.

Here are the fields you’ll meet most often:

Property	What it holds	Example
UID	A unique ID so updates replace, not duplicate	abc123@lazytools.io
DTSTAMP	When the file was created (UTC)	20240101T000000Z
DTSTART	Event start	20240603T093000
DTEND	Event end (exclusive for all-day)	20240603T103000
SUMMARY	The event title	Team Sync
LOCATION	Where it happens	Room 4
RRULE	Repeat pattern	FREQ=WEEKLY;COUNT=5

A few subtleties are worth knowing. A time like 20240603T093000 with no trailing Z is floating local time — it happens at 9:30 on whatever clock the viewer’s device shows. Append Z (20240603T093000Z) and it’s fixed to UTC; add a TZID parameter and it’s pinned to a named zone. The RRULE in the example above (FREQ=WEEKLY;COUNT=5) produces five weekly occurrences; swap COUNT for UNTIL=20240930T000000Z to repeat until a date instead.

The three things hand-writing gets wrong

• Text escaping. Commas, semicolons and backslashes are structural in iCalendar, so a title like Lunch, then demo must be written Lunch\, then demo. Line breaks become \n.
• Line folding. Lines longer than 75 octets must be wrapped onto continuation lines that start with a space. Skip this and strict parsers reject the file.
• The exclusive all-day end. For an all-day event, DTEND is the day after it ends. A single day on June 25 is DTSTART;VALUE=DATE:20250625 and DTEND;VALUE=DATE:20250626. This off-by-one is the single most common .ics bug.

The ICS generator handles all three automatically.

↕ convert

.csv — spreadsheet Full Name,Email,Phone Ada Lovelace,ada@example.com,…

The .vcf contact file

A vCard is even simpler — a BEGIN:VCARD … END:VCARD block per person:

BEGIN:VCARD
VERSION:3.0
FN:Ada Lovelace
N:Lovelace;Ada;;;
EMAIL;TYPE=INTERNET:ada@example.com
TEL;TYPE=CELL:+1-555-0100
END:VCARD

FN is the display name; N is the structured name (Last;First;Middle;Prefix;Suffix); EMAIL and TEL can repeat with type parameters. One .vcf file can hold one contact or your entire address book, stacked block after block.

Two version numbers dominate in the wild. vCard 3.0 (RFC 2426) is what most phones export today and is the safest for broad compatibility; vCard 4.0 (RFC 6350) is the current standard and adds cleaner handling of things like time zones and multiple addresses. The TYPE parameter labels each value — TEL;TYPE=CELL, EMAIL;TYPE=WORK, ADR;TYPE=HOME — so an app knows which number is the mobile and which address is home. Common properties beyond name and contact details include ORG (organization), TITLE (job title), ADR (a structured postal address), BDAY (birthday), URL and NOTE.

The two formats are close cousins — both descend from the same 1990s vCard/vCalendar lineage, so they share the escaping and line-folding rules while describing very different things:

Aspect	.ics (iCalendar)	.vcf (vCard)
Standard	RFC 5545	RFC 6350 (v4), RFC 2426 (v3)
Describes	Calendar events, to-dos	People and organizations
Top container	BEGIN:VCALENDAR	BEGIN:VCARD
Repeats as	VEVENT blocks	VCARD blocks
Common apps	Apple/Google Calendar, Outlook	iPhone, Android, Google Contacts
Classic gotcha	Exclusive all-day DTEND	FN vs structured N mismatch

VCF ↔ CSV: contact migration

Phones and address books speak .vcf; spreadsheets and many CRMs speak CSV. Moving between them is the core of contact migration:

• VCF → CSV turns each vCard into a spreadsheet row (name, email, phone, organization) — for editing, deduping or importing into a CRM.
• CSV → VCF turns each row back into a vCard — to load a spreadsheet of contacts onto a phone or into Outlook/Google Contacts.

The conversion has to unfold wrapped lines, decode vCard escaping, and quote CSV values with commas — the same class of details as the .ics format.

Why do this in the browser

Both file types are personal. An .ics can contain a private meeting link or your home address; a .vcf contains other people’s names, numbers and emails — data you have a duty to handle carefully. Uploading either to a random converter hands that data to a stranger. Every LazyTools tool here builds and converts the files in your browser, so they never leave your device and work offline.

The bottom line

.ics and .vcf are simple text formats with sharp edges: escape the special characters, fold long lines, and remember the all-day end date is exclusive. Generate calendar events with the ICS generator and migrate contacts with the VCF→CSV and CSV→VCF converters — all locally, because event and contact data is exactly the kind you shouldn’t hand to someone else’s server.