Blog Post Length & Depth: Which Prompt for Which Goal

A personal-finance blog about index investing publishes twice a week. One post is a 900-word note on why an expense ratio of 0.03% quietly beats one of 0.75% over thirty years. The other is a 4,000-word pillar on building a three-fund portfolio. Both are drafted with the same instruction, “write a blog post about X,” and the owner keeps meeting the same failure: the short post wanders into tangents, and the long post reads like the short one stretched with filler. Length is not a number you bolt on at the end. It is a decision you make before the first line.

This guide splits blog depth into four tiers, then gives a decision table, the moment each tier fits, a copy-paste prompt for each, and the trade-off you accept by choosing it. Every example stays inside one niche: a personal-finance blog that explains index investing to beginners. To fold these tiers into a repeatable editorial routine, see the ChatGPT blog post workflow. For a wider bank of drafting instructions you can reuse across formats, see ChatGPT writing prompts.

Depth Options at a Glance

Two questions pick the row. First, what should the reader do after reading: make one small decision, or understand a system? Second, will other posts link back to this page as the definition of the topic? One decision with no reference role is a snapshot. A system your own blog will cite again is a pillar. Most weak posts come from answering only the first question and defaulting to “a bit of everything.”

Matching Depth to Purpose

Snapshot for a single decision. A post titled “ETF or index fund for a beginner in Canada?” serves a reader with one question and five minutes. It states the answer in the first paragraph, gives three reasons, adds one worked dollar figure, and stops. The trap is drifting into market history or tax nuance that belongs in another post. If you find yourself typing a second subheading, you probably chose the wrong tier.

Explainer for one concept. “What is an index fund, actually?” needs room to define terms, walk through one concrete portfolio, and pre-empt the two most common misunderstandings. Twelve to eighteen hundred words lets you show a small table of holdings, explain the fee drag with a single number, and close with what to do next. This is the workhorse tier for a niche blog, and it is where most beginner traffic arrives.

Pillar for a topic you intend to own. A page called “The three-fund portfolio: a complete guide” has to cover asset allocation, fund selection, rebalancing, tax placement, and the failure modes of the approach. It earns its length by being the page every other post links to. Do not attempt a pillar without at least three original examples, one comparison, and a reason a reader would bookmark it rather than skim it.

Series when the topic refuses to compress. “How to build a retirement portfolio” is really five posts: risk tolerance, asset allocation, fund choice, contribution mechanics, and annual review. Split it, link each part to the hub, and give every part its own snapshot-sized job. A forced pillar of 8,000 words loses readers at the halfway mark; a series keeps each session short and every post indexable on its own.

Prompt Blocks by Post Type

Snapshot

Write a 750-word blog post for a personal-finance blog about index investing.
Question to answer: "Is a broad market index fund better than picking five individual stocks for a beginner?"
Rules:
- Answer in the first two sentences, then justify.
- Give exactly three reasons, each in its own short section.
- Include one concrete dollar example using a 10,000 investment over 20 years.
- Do not discuss taxes or retirement accounts; those belong in another post.
- End with one action the reader can take this week.
- Plain language, no hype, no exclamation marks.

Explainer

Write a 1,500-word explainer for beginners on what an index fund is.
Audience: someone who has heard the term but has never bought one.
Structure:
- A two-sentence plain definition.
- How the fund tracks an index, using one named example category.
- A small table of three sample holdings with weights.
- One worked example of fee drag across 25 years.
- Two common misunderstandings, each corrected in a short paragraph.
- A closing "what to do next" with one next step.
Rules:
- Define every term the first time it appears.
- Keep sentences short and avoid jargon such as "beta" unless you explain it.
- No promises about returns.

Pillar

Write a 3,200-word pillar page titled "The three-fund portfolio: a complete guide" for a personal-finance blog.
Cover, in this order: what the three funds are, why simplicity works, how to set target weights, how to pick the specific funds, how to rebalance, where to hold each fund for tax efficiency, and the four most common mistakes.
Rules:
- Every section must include at least one concrete number or example, not just description.
- Include one comparison table of two candidate fund sets.
- Add an FAQ of five questions at the end.
- Link conceptually to shorter posts on each subtopic instead of repeating them.
- Keep the tone calm and educational; no forecasts.

Series plan

Turn the topic "how to build a retirement portfolio" into a five-post series for a personal-finance blog about index investing.
For each post, output: a working title, the one question it answers, a target word count, and the single action the reader takes after reading.
Rules:
- No two posts may answer the same question.
- Order the posts so each one only needs the prior post as background.
- Mark which post should be the hub that all others link to.
- Keep each post between 900 and 1,600 words.
- Return a numbered list only.

Trade-Offs and Sequencing

Depth costs time to produce and attention to consume. A snapshot is cheap to write and easy to rank for a narrow query, but it rarely earns links. A pillar earns links and becomes the page your other posts point to, but it demands original material; a padded pillar is worse than three honest snapshots, because readers bounce and search engines learn to discount the page. Series spread the effort and keep each session short, but they only work once the hub and the internal links exist.

The practical sequence for a small blog is to start with explainers, because they match beginner intent and are short enough to publish weekly. Promote the best performer into a pillar once you have data on what readers actually ask, and break off the parts that draw the most questions into their own snapshots. Give the model a word range and a structure every time; a model told “roughly 1,500 words with five sections” produces far less filler than one told only the topic. Then edit for the one thing the model cannot supply: your own examples and the specific numbers from your niche.

Finally, match the length to the promise in the title. A title that asks a single question should be answered in the first paragraph, not the fourth. A title that claims a complete guide must actually cover the obvious subtopics, or readers will leave to find the one you skipped. Decide the tier, state the structure in the prompt, and let the word count fall out of the goal instead of setting it first.

Sources & Notes

Prompts tested with ChatGPT on the GPT-4o and GPT-4.1 models in October 2026; the same depth instructions work in Claude and Gemini. The personal-finance blog, its index-investing niche, and every dollar figure are illustrative teaching examples, not investment advice, and no fund, ticker, or return is real. Word-count tiers reflect common editorial practice rather than any platform rule. All prompts are copy-paste text; no external services were called. Category mapping follows the guunn.com AI Prompts plan (Writing = 956).