3 ways to "Guess-timate" the Yarn Needed
Yarn Estimator Tool
The Most Accurate Method
Stitch Types
Knit it Now Patterns

How Much Yarn do I Need?

"How much yarn do I need for a woman's XLarge / 1X tank top or shell?"

There isn't a simple answer for that question, Keri! 

Immediately, we started asking more questions.
  • How long do you want the tank?
    You may be short and want a high hip length or you may be tall and need more length

  • XLarge / 1 X can mean a wide range of bust dimensions
    Better to know the desired FINISHED bust size

  • What yarn are you using?
    Standard gauge? Mid-Gauge? Bulky?

  • What stitch pattern are you using?
    Tuck will take TONS more yarn than lace 

  • What's YOUR stitch gauge?
    The only accurate way to estimate yarn needed is to knit and weigh a swatch

3 ways to "guess-timate" how much yarn you need:

Use "The Knitter's Handy Guide to Yarn Requirements".  This inexpensive, classic leaflet helps you determine "how much yarn?" The included charts are for a number of knitting shapes (hats, vests, sweater, mittens).  Keri could use the "vest" chart.  She would choose the stitch gauge of the yarn she is planning to use and select a vest by bust size.  The chart will then give her an idea of how many yards/meters she will need.
BUT .... it doesn't take into account the desired length

Search Ravelry or other pattern sources for a similar sweater shape, knit in a yarn of similar weight as you plan to use.  Examine the sizing, stitch pattern, length and use their yarn estimate for your project.
BUT ... This can be time consuming and simply guess work

For the MOST ACCURATE way to determine how much yarn you need, check out the resources at Knit it Now!  

Resources at Knit it Now

In the store: Classic guide based on stitch gauge and garment shapes

Knitters Handy Guide Leaflet
Use YOUR gauge and measurements to estimate yarn needed for each piece of your garment

The MOST ACCURATE method of calculating yarn usage

Certain stitch types use more yarn than others.
Compare Tuck, Slip, Lace and Stockinette

DesignaKnit to the rescue.
DAK will even calculate yardage for EACH COLOR in a design