Figure

HVAC

Duct offsets

Calculate wrapper and radius results, compare an offset change, and divide bend angles into elbows.

The Offset mode uses straight length, vertical shift and opening height to calculate an OGEE fitting’s related dimensions. Diagonal slant length is not the straight-length input. Label these measurements before comparing wrappers, radii or elbow angles.

Start with the three dimensions shown in the fitting sketch: actual length is the straight distance between the ends, offset is their vertical shift, and height is the opening’s vertical size. For a round profile, the opening height is its diameter. An OGEE wrapper follows the fitting’s curves, so its length is not just the diagonal between the ends.

The first example uses 10 ft actual length, 5 ft offset and 7 ft height. It reports a 6 ft 3 in radius and an 11 ft 7⅛ in wrapper. The later examples show fractional and metric entry, changes to the offset, and how the bend angle is divided into elbows. Keep each result’s caption with its value.

Find offset dimensions

Use actual length, offset and height.1

Measure the fitting

Length: straight distance between the ends. Offset: vertical shift. Height: vertical size of the opening. For a round profile, use its diameter. Use the straight length, not the diagonal slant.

Offset fitting: Length (L) is the straight horizontal distance between the ends, Offset (O) is the vertical shift between their bottom edges, and Height (H) is the vertical size of the left opening. Slant length is diagonal; do not enter it as Length.LHOslant
Schematic, not to scale.

Start a new calculation

Use 2nd if a unit is above its button.

  1. Start fresh

    Press Clear entry, then Clear entry.

  2. Press the keys

    Press Select length units.

Open Offset

Skip this step if Offset is already open. Skip Close if no mode is open.

  1. Press Close calculation mode, then Open Offset.

1. Actual length

  1. Actual length

    Press 10, then feet, then Store length.

2. Offset

  1. Offset

    Press 5, then feet, then Store offset.

3. Height

  1. Height

    Press 7, then feet, then Store height.

4. Calculate the centerline radius

  1. Press Equals.
  1. Example · Centerline radius6 feet 3 inches

5. Read wrapper length

  1. Press Equals.

Results in orderPress = to continue

  1. Example · Wrapper length11 feet 7 and 1/8 inches
  2. Heel radius9 feet 9 inches
  3. Throat radius2 feet 9 inches
  4. Angleabout 26.57°
  5. Entered length10 feet
  6. Entered offset5 feet
  7. Entered height7 feet
  8. Back to centerline radius6 feet 3 inches
  9. Wrapper length11 feet 7 and 1/8 inches

Change or leave the calculation

Press = to read the next result. To change an input, enter its value and tap its key, then =. One C keeps the inputs; C twice clears them. Tap Close to leave this mode.

Use fractional inches

Use actual length, offset and height in inches.1

Measure the fitting

Length: straight distance between the ends. Offset: vertical shift. Height: vertical size of the opening. For a round profile, use its diameter. Use the straight length, not the diagonal slant.

Offset fitting: Length (L) is the straight horizontal distance between the ends, Offset (O) is the vertical shift between their bottom edges, and Height (H) is the vertical size of the left opening. Slant length is diagonal; do not enter it as Length.LHOslant
Schematic, not to scale.

Start a new calculation

Use 2nd if a unit is above its button.

  1. Start fresh

    Press Clear entry, then Clear entry.

  2. Press the keys

    Press Select length units.

Open Offset

Skip this step if Offset is already open. Skip Close if no mode is open.

  1. Press Close calculation mode, then Open Offset.

1. Actual length

  1. Whole inches

    Press 45, then inches, then Store length.

2. Offset

  1. Whole inches

    Press 12, then inches.

  2. Top number

    Press 5, then Fraction.

  3. Bottom number

    Press 8, then Store offset.

3. Height

  1. Inches

    Press 38, then inches, then Store height.

4. Calculate the centerline radius

  1. Press Equals.
  1. Example · Centerline radius43 and 1/4 inches

5. Read wrapper length

  1. Press Equals.

Results in orderPress = to continue

  1. Example · Wrapper length47 and 5/16 inches
  2. Heel radius62 and 1/4 inches
  3. Throat radius24 and 1/4 inches
  4. Angleabout 15.67°
  5. Entered length45 inches
  6. Entered offset12 and 5/8 inches
  7. Entered height38 inches
  8. Back to centerline radius43 and 1/4 inches
  9. Wrapper length47 and 5/16 inches

Change or leave the calculation

Press = to read the next result. To change an input, enter its value and tap its key, then =. One C keeps the inputs; C twice clears them. Tap Close to leave this mode.

Use millimeters

Use actual length, offset and height in millimeters.1

Measure the fitting

Length: straight distance between the ends. Offset: vertical shift. Height: vertical size of the opening. For a round profile, use its diameter. Use the straight length, not the diagonal slant.

Offset fitting: Length (L) is the straight horizontal distance between the ends, Offset (O) is the vertical shift between their bottom edges, and Height (H) is the vertical size of the left opening. Slant length is diagonal; do not enter it as Length.LHOslant
Schematic, not to scale.

Start a new calculation

Use 2nd if a unit is above its button.

  1. Start fresh

    Press Clear entry, then Clear entry.

  2. Press the keys

    Press Select length units.

Open Offset

Skip this step if Offset is already open. Skip Close if no mode is open.

  1. Press Close calculation mode, then Open Offset.

1. Actual length

  1. Millimeters

    Press 2045, then millimeters, then Store length.

2. Offset

  1. Millimeters

    Press 573, then millimeters, then Store offset.

3. Height

  1. Millimeters

    Press 1727, then millimeters, then Store height.

4. Calculate the centerline radius

  1. Press Equals.
  1. Example · Centerline radius1967.868 mm

5. Read wrapper length

  1. Press Equals.

Results in orderPress = to continue

  1. Example · Wrapper length2150.408 mm
  2. Heel radius2831.368 mm
  3. Throat radius1104.368 mm
  4. Angleabout 15.65°
  5. Entered length2045 mm
  6. Entered offset573 mm
  7. Entered height1727 mm
  8. Back to centerline radius1967.868 mm
  9. Wrapper length2150.408 mm

Change or leave the calculation

Press = to read the next result. To change an input, enter its value and tap its key, then =. One C keeps the inputs; C twice clears them. Tap Close to leave this mode.

Compare two offsets

Compare two offsets while keeping actual length and height.1

Start a new calculation

Use 2nd if a unit is above its button.

  1. Start fresh

    Press Clear entry, then Clear entry.

  2. Press the keys

    Press Select length units.

Open Offset

Skip this step if Offset is already open. Skip Close if no mode is open.

  1. Press Close calculation mode, then Open Offset.

1. Enter the first fitting

  1. Actual length

    Press 52, then inches, then 5, then Fraction, then 8, then Store length.

  2. First offset

    Press 15, then inches, then 3, then Fraction, then 16, then Store offset.

  3. Height

    Press 34, then inches, then Store height.

  4. Complete the value

    Press Equals.

Results in orderPress = to continue

  1. Example result49 and 3/8 inches
  2. Wrapper55 and 1/2 inches
  3. Heel66 and 3/8 inches
  4. Throat32 and 3/8 inches
  5. Thetaabout 16.10°

2. Replace only the offset

Keep Length and Height. Enter the new offset, tap Offset, then =. Do not press C twice.

  1. New offset

    Press 25, then inches, then 3, then Fraction, then 16, then Store offset.

  2. Complete the value

    Press Equals.

Results in orderPress = to continue

  1. Centerline radius33 and 13/16 inches
  2. Wrapper60 and 5/16 inches
  3. Heel50 and 13/16 inches
  4. Throat16 and 13/16 inches
  5. Thetaabout 25.58°
  6. Stored length52 and 5/8 inches
  7. Stored offset25 and 3/16 inches
  8. Stored height34 inches
  9. Centerline radius again33 and 13/16 inches

Keep in mind

The longer wrapper belongs to the larger offset. For a transitioning cheek with a slant length, establish the corrected true length before applying this offset workflow.

Four elbows or two

Use actual length, offset and height. Read the angle for four elbows; double it for two.

Start a new calculation

Use 2nd if a unit is above its button.

  1. Start fresh

    Press Clear entry, then Clear entry.

  2. Press the keys

    Press Select length units.

Open Offset

Skip this step if Offset is already open. Skip Close if no mode is open.

  1. Press Close calculation mode, then Open Offset.

1. Enter length, offset and height

  1. Actual length

    Press 80, then inches, then 1, then Fraction, then 2, then Store length.

  2. Offset

    Press 22, then inches, then 9, then Fraction, then 16, then Store offset.

  3. Height

    Press 68, then inches, then Store height.

2. Find the angle for four elbows

Angle format
Degrees, minutes, seconds
  1. Press Equals, then Equals, then Equals, then Equals, then Equals.
  1. Example resultDMS 15.39.26

3. Double it for two elbows

  1. Multiply

    Press Multiply.

  2. Enter 2

    Press 2, then Equals.
  1. Example resultDMS 31.18.51

Keep in mind

DMS shows degrees, minutes and seconds. Multiplication uses the unrounded angle; doubling rounded seconds by hand may differ.

Display assumptions

  1. Fraction precision: 1/16″; Fraction format: Simplified. ↩1↩2↩3↩4

Keep the guide beside your calculation.

These examples are built into Figure. Open Help while you calculate, then return to the same guide and reading position.

See Help in Figure →